Popular Science Monthly/Volume 55/July 1899/The Use of Acetylene



IT is now five years since the use of acetylene as an illuminant was suggested to the public, and it may be of interest to give a sketch of what has been done during this time, especially as it seems that with the year 1899 the tentative period which must characterize every new industry is in some respects passed, and a period of solid and well-directed industrial effort, backed by ample capital, has begun. The knowledge gained during this tentative period by the laboratory experiments of scientific men, and by the practical work of inventors and promoters, has made it possible for the industry to enter on its new phase. To understand its present and to foresee its future importance it is necessary briefly to review the work of the last years.

In May, 1892, Mr. Thomas Willson, a Canadian electrician, tried to make the metal calcium in an electric furnace in his works at Spray, North Carolina, by heating a mixture of lime and coal dust. He thought that the lime (calcium oxide) would act on the coal (carbon) to form calcium and carbon monoxide. He did not succeed in getting calcium, but found in the furnace a brown, crystalline mass, which was decomposed by pouring water on it, yielding an inflammable gas. Willson is not a chemist, and he therefore sent specimens of the material to several men of science to determine its nature. It was shown to be calcium carbide, a compound of calcium and carbon, formed by the action of the carbon on the calcium oxide. The reaction expressed in chemical symbols is CaO + 3C = CaC2 + CO. The gas formed by the action of water was acetylene, a compound of carbon and hydrogen. The reaction is CaC2 + H2O = C2H2 + CaO; calcium carbide and water form acetylene and lime. If water enough is added, the lime is slaked, and slaked lime, or calcium hydroxide, Ca(OH2), is formed. Neither calcium carbide nor acetylene was a new discovery; acetylene was discovered by Edmund Davy in 1836, and its properties were studied by Berthelot in 1862. Impure calcium carbide was first made in 1802 by Wöhler, who described its decomposition by water into acetylene and lime. What was there new, then, in Willson's discovery? Two important facts: (1) He was the first to make carbide by a method applicable commercially; (2) he was the first to make crystalline carbide. Wöhler's carbide was impure and amorphous; Willson's, nearly pure and crystalline, so that he succeeded in obtaining United States patents for crystalline carbide, and, as all carbide made by commercial processes is crystalline, its manufacture is covered by Willson's patents.

In the same year, 1892, Prof. Henri Moissan, of Paris, announced the discovery of crystalline calcium carbide. Moissan's discovery, too, was an accidental one. He was reducing refractory metallic oxides in an electric furnace made of lime. At the close of the article in which he reports his work to the French Academy of Sciences (Comptes Rendus de l'Academie Française, vol. cxii, page 6, December 12, 1892) he refers in two lines to the formation of an ill-defined carbide of calcium by the action of the carbon electrodes on the lime of which his furnace was made.

As is common with most important inventions, there is a dispute as to the priority of making carbide by an electric furnace; and the wonder is, not that there is a dispute, but that there are so few claimants. A few words of explanation of the electric furnace will show why. The enormous heat of the electric furnace (2000° to 3000° C.) is caused by an electric arc, formed by currents playing between carbon electrodes; carbon is often used in the furnace processes; here we have one constituent of calcium carbide. Lime, the material for the other constituent, withstands heat better than any other common substance excepting magnesia: naturally, inventors would use it, as Moissan did, as a refractory lining to the furnace. Electric furnaces were not new. The conditions then were such that the discovery of the carbide was fairly forced on experimenters, and, as we have seen, the discoveries of Willson and Moissan were both accidental.

American priority was claimed by Willson, French priority by the friends of Moissan, German priority by Professor Borchers, of Aix-la-Chapelle. Fortunately for Willson, among those to whom he had sent specimens of carbide was Lord Kelvin, the famous English physicist, whose reply to Willson, stating that the substance received was calcium carbide, was dated October 3, 1892, two months before Moissan's first publication. Borchers's claims are too vague to waste space on. Willson's priority is now generally recognized excepting in France. The German Government has acknowledged it, and has annulled the German patent granted to Bullier.

Commercial carbide is essentially an American discovery, and it was developed industrially by Willson's associates before industrial action began abroad. Messrs. Dickerson and Suckert, of New York, were the first to undertake the industrial liquefaction of acetylene. Dr. G. de Chalmot, chemist, and Mr. J. M. Morehead, electrician, worked up the details of the furnace process in the early days at Spray, North Carolina, and the purity and the yield from a given weight of material of their carbide have never been excelled, though cheaper working furnaces are now in use.

Carbides of other metals can be made in the electric furnace, but, owing to the cheapness of the new material, calcium carbide is the only one of these which has industrial value as a source of acetylene. One pound of pure carbide yields 5.89 cubic feet of acetylene.

Thus far carbide has been found industrially valuable for two other purposes. The one is for carbonizing steel; experiments in Germany show that iron or soft steel takes up carbon more readily when it is heated with carbide than when it is heated with coal dust or charcoal. Some steel works are now using carbide for this purpose. The other use of carbide is more important. It is found to be a valuable germicide. It is said to be the most effectual preventive of black rot, and to destroy the Phylloxera, the two worst enemies of the grape. The action of the carbide as a germicide depends on its decomposition by the moisture of the soil, forming acetylene, which kills the Phylloxera. If the use of carbide on a large scale substantiates the claims made for it, this is a discovery of vast importance. The ravages caused by the Phylloxera in the vineyards of southern Europe, of Africa, and Australia must be ranked as great national calamities.

A temperature ranging from 2000° to 2500° C. (3600° to 4500° Fahrenheit) is required to make carbide. It is probable that this temperature can be economically attained only by the electric furnace using water power as the source of the electric current, and this is the only method used for making carbide, with the exception of the Walther process, which does not use electricity but depends on the intense heat generated by burning acetylene under pressure. In electric furnaces the formation of carbide depends simply on the heat of the arc, which fuses the mixture of lime and coke. The latest improvements on the first very simple forms of furnace have secured continuity of work and economy of electric energy. In the United States carbide is made exclusively in the Horry furnace. This furnace consists of a huge short cylinder or hollow wheel, mounted to revolve slowly on a horizontal shaft. PSM V55 D354 Horry furnace showing the electrodes.pngHorry Furnace, showing Electrodes. The periphery of the cylinder is closed by removable cast-iron slats. As the cylinder is partly revolved on its axis from time to time, the slats are taken off from one side and replaced on the other, thus leaving the top always open. The cylinder is filled on one side with the powdered mixture of coke and lime. Into the mixture two vertical carbon electrodes project downward through the open top of the cylinder. As the carbide is formed, the cylinder is revolved, lowering the mass from the electrodes. The fused carbide cools, hardens, and is broken off and removed as it rises on the other side of the slowly revolving cylinder; new material is constantly fed in to maintain the level around the electrodes. The process in the Horry furnace is continuous; the furnace can be run without arresting the current until repairs are necessary. It is said to combine the different theoretical improvements referred to, and to reduce the cost of production. The Horry furnace is in use at Niagara Falls and at Sault Ste. Marie. At St. Catherine's, Canada, Willson is using his own furnace. Abroad, the older types of furnace, the Willson, Bullier, and Héroult, are those chiefly in use.

The actual ingot of good commercial carbide is nearly pure—ninety-six to ninety-nine per cent—but the ingot is surrounded by a crust of carbide mixed with unchanged material, containing forty to

PSM V55 D355 Horry furnace showing gearing.pngHorry Furnace, showing Gearing. seventy per cent of carbide. Foreign makers break and blend ingot and crust to standard size, the best makers guaranteeing their carbide ninety per cent pure, giving five cubic feet of acetylene per pound (pure carbide gives 5.89 cubic feet). Eight to nine pounds of carbide per horse power in twenty-four hours, averaging five cubic feet of acetylene, is considered satisfactory work. The Union Carbide Company, which controls the sale of carbide in the United States, is selling graded carbides under guarantee, the first grade being the nearly pure ingot, the lower grade the crust.

As the moisture of the air decomposes the carbide, it must be broken up as soon as made, and packed in air-tight tin cans, varying in size from one to four hundred pounds. PSM V55 D355 Horry furnace with just removed carbide.pngHorry Furnace,
showing Carbide just removed.
The present price of carbide abroad averages $96.80 in large lots, and $7.26 per hundredweight in small lots, packing included; in the United States, $70 per ton in large lots, and $4.50 per hundredweight in small lots, packing included. In 1898, 4,650 tons are said to have been made in the United States and Canada, and a much larger amount abroad. The output for 1899 is estimated at 12,000 tons for the United States, with a capacity in the new works in erection at Sault Ste. Marie and at Niagara Falls of 41,000 tons. The new works building in Europe, to be finished in 1899-1900, have a capacity for making 80,000 metric tons. These figures will justify the statement made at the beginning of this article, that the new industry has found ample capital.

The statement is still current that acetylene attacks copper and brass, forming an explosive compound. This is not true. Exhaustive experiments by Moissan and by Gerdes, keeping these and other metals in contact with acetylene for months at a time, have shown that the metals were not affected. The conditions under which the explosive copper acetylide is made in laboratories can not well occur in generators or gas holders. It has been said that acetylene is very poisonous; the experiments of many observers, and especially those of Gréhant, do not confirm this statement. Gréhant experimented on dogs, causing them to breathe mixtures of acetylene, air, and oxygen, which always contained 20.8 per cent of oxygen, this being the percentage of oxygen in pure air. By this device he was able to discriminate between the poisoning caused by acetylene and suffocation caused by insufficient oxygen. A mixture containing twenty per cent acetylene inhaled for thirty-five minutes did not seem to trouble the animal. A sample of the dog's arterial blood contained ten per cent of acetylene. A dog which inhaled a mixture containing forty per cent of acetylene died suddenly after fifty-one minutes, having inhaled one hundred and twelve litres of the mixture; the arterial blood contained twenty per cent acetylene. Gréhant proved that acetylene simply dissolves in the blood plasma, while carbon monoxide forms a compound with the hæmoglobin of the blood. A dog breathing a similar mixture of air, oxygen, and illuminating gas containing only one per cent of carbon monoxide quickly showed convulsive movements, and died after ten minutes; its blood contained twenty-four per cent of carbon monoxide. Thus acetylene, while slighthy poisonous, is less poisonous than coal gas, and vastly less than water gas, which contains a high percentage of carbon monoxide.

A pressure of thirty-nine atmospheres and three quarters at 20° C. converts acetylene into a liquid weighing one third as much as the same volume of water, while one cubic foot of the liquid when released from pressure gives five hundred cubic feet of gas.

Hitherto acetylene is used only as a source of heat or as a source of light; yet with very cheap carbide it would prove useful in many ways in chemical industry, and its use would have the most widespread effect on industry and agriculture. For instance, a method of making alcohol from acetylene is patented abroad, and by another patented process it is proposed to make sugar from acetylene. With the present prices of alcohol, sugar, and carbide, these processes have no commercial value.

Acetylene may be made from the carbide in gas works and delivered to the consumer through mains like ordinary illuminating gas; or it may be liquefied at a gas works and delivered to the consumer in the liquid form under pressure; or the consumer may purchase carbide and generate acetylene for his own consumption. All three of these methods are in use.

To understand the attitude of insurance companies and of consumers toward liquid acetylene it will be well to examine its record for the last few years. Those interested in methods for liquefying acetylene, and for reducing the pressure of the liquid at the place of consumption so that the consumer actually uses it as a gas under a water pressure of six inches or less, may find processes described in detail in the Progressive Age, and in other technical journals. Suffice it to say that the methods in use in this country and abroad are simple and effective. The purified acetylene is delivered in strong steel cylinders, which may be placed in a special building or case and need not be handled by the consumer. It has been proved by the exhaustive experiments of the eminent French chemist Berthelot that liquefied acetylene in cylinders can not be exploded by blows or shocks to the closed cylinder. If it is exploded, however, by causing a spark within the cylinder, the explosive force is very great, being about equal to that of gun cotton.

The use of the liquefied acetylene is so simple and clean that the attention of inventors was first turned to this mode of supply. It may in future come again into prominence despite the present strong feeling against it, its use in many cities being prohibited. This feeling was caused by a number of explosions, accompanied by loss of life. Three of these explosions occurred in factories for liquefying acetylene; one in a factory where liquid acetylene regulators were made; several in buildings of consumers. In October, 1896, Pictet's works in Paris were wrecked by the explosion of a cylinder filled with liquid acetylene; evidence proved that the cylinder was held in a vise, and that the two workmen killed were at the ends of a wrench, closing or opening the valve, supposing the cylinder to be empty. The explosion was caused either by a spark from friction in turning the screw, or by the too sudden opening of the valve and releasing the pressure, causing a shock sufficient to decompose the liquid. In December, 1896, the works of G. Isaac, in Berlin, were destroyed by an explosion in the condenser where the cooled acetylene was liquefied by pressure; Isaac and three workmen were killed. Evidence showed that through carelessness warm water instead of cold water was in contact with the condenser, thus warming the liquid and increasing the pressure to a point which burst the condenser. In December, 1897, the works of the Dickerson & Suckert Acetylene Gas Liquefying and Distributing Company in Jersey City were destroyed by fire caused by the explosion of a cylinder filled through carelessness of workmen with a mixture of air and liquid acetylene—i.e., with an explosive mixture—killing the superintendent and a workman. In the explosion at the regulator factory at New Haven, January, 1895, the valve of the cylinder, on which one of two workmen killed was working, broke; a large volume of acetylene escaped and ignited from a lighted candle. In all four cases the explosions were caused by ignorance or carelessness incident to the beginnings of a new industry, and could be avoided by experience and skill.

It should be stated that in the explosion at Paris all of the full acetylene cylinders were dug out of the ruins unhurt. The same was true at Berlin, where five full cylinders were blown against the wall of the building by the explosion of the condenser, but did not explode. At Jersey City sixty filled cylinders were exposed to the heat of the fire following the explosion; they were fitted with safety diaphragms of fusible metal; forty-eight remained intact, the acetylene burning off quietly as it escaped through the fused diaphragm, and twelve exploded, either on account of imperfection of the diaphragms or stoppage of the air passage leading from the diaphragm. The explosions of liquid acetylene in buildings of consumers have been due in every case to gross carelessness and ignorance on the part of the consumer.

Although one of the chief points in favor of the liquid acetylene is its portability, yet it can be shown that it is still easier to carry carbide to the consumer. One cubic metre of acetylene is compressed to two litres in liquid form; two litres of carbide weigh 4.44 kilogrammes, which will produce a cubic metre and a third of acetylene, reckoning three hundred litres to the kilogramme, which is the average guaranteed yield of carbide. The light tin carbide cans occupy less space and weigh less than the heavy steel cylinders, while the generation of the gas is simple and, with proper generators, perfectly safe. On the other hand, the generators must be cared for, must often be filled with fresh carbide, and from time to time must be cleaned. With the generator system acetylene is as safe as or safer than illuminating gas. Berthelot has shown that at pressures below two atmospheres a vessel filled with acetylene can not be exploded by the explosion of a cap of fulminating mercury within the vessel, nor by heating a wire which extends into the vessel to a white heat by an electric current. The reason is that the acetylene can not explode unless it is decomposed into its elements, carbon and hydrogen; to decompose it requires a certain amount of energy. While the energy of the glowing wire or of the exploding cap causes a local decomposition at the point of contact, it is not sufficient to spread the decomposition further. Acetylene forms an explosive mixture with air; so does illuminating gas. The odor of acetylene is unpleasant; so is the odor of the water gas used generally in the United States, and the acetylene can be cheaply deodorized.

As the generator system, then, is the general one, the most important question to the consumer is what generator to buy, and it is a perplexing question. The carbide manufacture is so organized that it is everywhere under the control of powerful and responsible companies which sell a guaranteed product. The burners now in use are nearly all good. With generators it is different; the market is flooded with them at all prices, ranging in value from worse than useless to very good, as regards safety, economy, and quality of light. As the generator question is by far the most important and the least understood in the whole acetylene industry, it will be well to give a full account of the results of the experiments which have been made within the last two years on this question. The most exhaustive experiments are those of the English expert, Professor Lewes, and his results agree with those of other observers.

Lewes first determined the amount of heat developed by the decomposition of carbide by water, and the conditions which tend to lessen or increase the intensity of the reaction. The average result of the experiments as to the amount of heat was 446.6 calories for pure carbide, and a little less for commercial carbide (to state this differently, one pound of carbide, when decomposed by water, gives off heat enough to raise the temperature of 446.6 pounds of water 1° C, or to raise the temperature of one pound of water 446.6° C). As the intensity of the heat developed determines the highest temperature attained during the decomposition, and is a function of the time needed to complete the action, and as the decomposition of carbide in contact with water is extremely rapid, it is evident that the temperature developed may be so high as to cause disaster. All the generators at present before the public may be classified under three heads: 1. Those in which water is allowed to drip or flow slowly on a mass of carbide, the evolution of the gas being regulated by the stopping of the water. 2. Those in which water in considerable volume is allowed to rise in contact with carbide, the evolution of the gas being regulated by the driving back of the water by the increase of pressure in the generating chamber. 3. Those in which the carbide is dropped or plunged into an excess of water.

The conclusions deduced from a large number of experiments were that when, as in type 1, water is allowed to drip or flow in a fine stream upon a mass of carbide, the temperature rapidly rises until after eighteen to twenty-five minutes the maximum is reached, which varies from 400° to 700° C. (720° to 1120° Fahrenheit), and it is probable that in some of the mass the higher limit is always reached, as traces of tar are usually found in the residual lime, in some cases in sufficient quantity to make the lime yellow and pasty, while vapors of benzene and other polymerization products pass off with the gas. Leaving the question of temperature in this type of generator, another important question is the length of time during which the generation of gas continues after the water supply is automatically cut off. It is found that gas is evolved with increasing slowness sometimes for an hour and three quarters after the water supply has ceased, the total volume of gas so evolved being large.

The experiments showed that in any automatic generator of this type the cut-off should be so arranged that one quarter of the total capacity of the gas holder is still available to store the slowly generating gas.

The second class of generators bring about contact either by water rising from below to the carbide suspended in the cage (II, A), or by a cage of carbide suspended in a movable bell which, as it falls, dips the carbide into water, withdrawing the carbide from the water as the excessive generation of gas lifts the bell (II, B). Lewes found that under certain conditions generators of the type II, B were far worse than those of type I.

The trials were made with a movable glass bell, with counterweights, containing a half-pound of carbide. The maximum temperatures reached in four trials were 703°, 734°, 754°, and 807° C. PSM V55 D360 Type 1 generator for carbide treatment.pngType 1.
Type of Generator.
Excessive heating took place in every case; in the last mentioned the temperature was far above the point at which acetylene is decomposed into carbon and hydrogen, a thin black smoke being formed immediately around the carbide while tar vapor poured out. On removing the residue after cooling it was found to be coated with soot and loaded with tar. On several occasions the charge was removed from the generator just after the maximum temperature was reached, and was found to be at a bright red heat.

These experiments are of the greatest practical importance. At 600° acetylene begins to polymerize—i. e., to form more complex hydrocarbons, which are liquid, or solid, at ordinary temperatures. Probably in the generator acetylene is first given off so rapidly that the heat does not act on it, but as decomposition advances into the

center of the mass of carbide, the acetylene generated has to pass through the external layers, which, as shown, may be at high temperatures, above that at which acetylene decomposes; thus a considerable amount of gas is lost, and the tar formed may distill into

PSM V55 D361 Types of carbide treatment generators.png
Type II, A. Type II, B. Type III.
Types of Generators.

the generator and tubes, clogging the tubes. A more serious evil is the deterioration in the illuminating quality of the gas. Samples of the gas were taken as the maximum temperature was approached, and analyzed with this average result: Acetylene, seventy per cent; other hydrocarbons, eleven per cent; hydrogen, nineteen per cent. This reduces the illuminating value from two hundred and forty to one hundred and twenty-six candles. The hydrocarbons consist largely of benzene, which requires three times as much air for complete combustion as acetylene does. The best possible acetylene burner smokes when the acetylene contains benzene.

At first sight these experiments would seem absolutely to condemn generators of class II, yet the fact remains that some excellent generators are of this type. Under certain conditions excessive overheating may be avoided. The rising bell shown in II, B should be discarded. Generators in which the water rises from below, and slowly attacks the carbide, can be made safe if the water is never driven back from the carbide, and the carbide is in separated layers as in II, A. Under these conditions the water is always in excess at the point where it attacks the carbide, so that the evaporation, by rendering heat latent, keeps the temperature down, the temperature of the melting point of tin, 228° C, being rarely reached in good generators where these conditions are met.

Undoubtedly the best generators, and the only ones which from a scientific point of view should be employed, are those of class III, in which carbide falls into an excess of water. In such generators it is impossible to get a temperature higher than the boiling point of water, 100° C, while with a properly arranged tank the temperature never exceeds that of the air by more than a few degrees. Under these conditions the absence of polymerization and the washing of the nascent and finely divided bubbles of gas by the limewater in the generator yield acetylene of a degree of purity unapproached by any other form of generator.

When acetylene is burned in air under such conditions that the flame does not smoke, it has been proved by Gréhant that there is no carbon monoxide among the combustion products; the acetylene combines with the oxygen of the air to form carbon dioxide and water (C2H2 + 5O = 2CO2 + H2O). One cubic foot of acetylene requires two and a half cubic feet of oxygen. Supposing a room to have an illumination equal to sixty-four standard candles; this amount of light from candles would use up 38.5 cubic feet of oxygen from the air, and would give off forty-three cubic feet of carbon dioxide; petroleum requires, in cubic feet, twenty-five of oxygen, and gives off forty of carbon dioxide; gas burned with a flat flame requires about twenty-five oxygen and gives nineteen carbon dioxide—with an Argand flame a little less, while with the Welsbach burner gas requires only three oxygen, and gives off 1.8 carbon dioxide; acetylene requires five oxygen and yields four carbon dioxide. So that, light for light, acetylene fouls the air less than any ordinary illuminant excepting the Welsbach gas burner. (With incandescent electric light there is no combustion and no fouling of the air.)

Under the best conditions five cubic feet of acetylene give a light of two hundred and forty candles for one hour, or we may speak of acetylene as a two-hundred-and-forty-candle gas. Yet this statement, though strictly true, may be misleading. When ordinary illuminating gas is tested with the photometer, it is burned from a standard flat-flame burner, burning five cubic feet per hour. Now the amount of light given by such a gas flame is no greater than is pleasant to the eye; it is true that if we burn five cubic feet of acetylene from a suitable flat-flame burner, a light of two hundred and forty candles is given, but it is unfair to take this ratio as representing the actual relative illuminating value of the two lights, because we neither need a light of two hundred and forty candles, nor is such an amount of light issuing from one burner endurable to the eye. One-foot or one-half foot acetylene burners are used for domestic lighting; light from the best one-foot burners averages thirty-two to thirty-five candles per cubic foot. With acetylene, as with every other illuminating gas, the smaller the burner and consumption, the less light per cubic foot of gas is obtained. Another important point is that while these figures represent the best practical illumination obtained from acetylene by the burners hitherto in use, the standard flat-flame burner does not give the best gaslight; with a good Welsbach burner a cubic foot of illuminating gas will give a seventeen-candle light as an average. The comparison, to be fair, should be between acetylene and the Welsbach light.

The reader will ask whether it is not possible to burn acetylene with other forms of burner, or to use it with Welsbach mantles. Successful acetylene burners of the Argand or of the regenerative type have not yet been introduced; but in Germany a new acetylene burner with Welsbach mantle promises good results. Experiments in England with an acetylene Bunsen burner and Welsbach mantle gave a light of ninety candles per cubic foot of acetylene used. It remains to be seen whether it is necessary to modify the composition of the mantles because of the intense heat of the acetylene Bunsen flame, which gives a temperature of 2100° to 2400° C. (3812° to 4397° Fahrenheit).

It would extend this article to undue length to speak of the various uses of acetylene as an enricher of other gases, but a mixture of acetylene and Pintsch oil gas now in use on all the Prussian state railways deserves mention, as it is a success, and ten thousand tons of carbide will be used this year for lighting cars by this system. Lewes's new invention of a very cheap methane water gas which is enriched by acetylene, carried to the consumer through mains, and burned in ordinary burners, is also promising.

Insurance and police regulations vary for every country. As a rule, restrictions are put on the use of liquid acetylene, and on the amount of carbide to be kept in storage. Generators must stand in separate buildings, which, in towns, must be fireproof.

The Willson patents cover the manufacture of crystalline carbide in the United States, Canada, and the South American states; and, as all carbide made by the electric furnace is crystalline, no carbide can be made independently of these patents in these countries.

In conclusion, it may be predicted that within the next few years acetylene will prove a factor in giving us an improved and cheaper light. Whether this will be an acetylene-Welsbach light or whether the acetylene will be chiefly used as an enricher of cheaper gases the future will show.

    Note.—We are indebted to the courtesy of the Electrical World and Engineer for cuts showing the Horry furnace.

    Note.—We are indebted to the courtesy of the Progressive Age for cuts showing types of generators.