Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/100

80 COAL [ANALYSIS. classification, 1 in 334, with a produce of about G5,000 tons of coal for each death. It would appear, therefore, that the proportional loss of life, in the collieries of the United Kingdom, is less than that in foreign countries. Analysis of Assay and Analysis. The chemical examination of a coal. C oal may be cither complete or partial. When it is desired to obtain information as to the exact composition, the analysis is conducted in the same manner as the analysis of organic compounds by combustion with oxide of copper or chromate of lead in a hard glass tube, the carbonic acid and water formed being absorbed by solution of hydrate of potassium and dry chloride of calcium respec tively, and the proportion of carbon and hydrogen being calculated from the increase of weight in the tubes con taining the absorbing media. It is usual to operate upon a small sample (about 5 grains), which is very finely powdered and placed in a small trough or boat of platinum in the tube, the combustion being aided by a stream of oxygen from a gasholder. By this means the incombustible residue or ash is left in a condition for weighing, being free from admixture of foreign substances. Sulphur is determined by the fusion of a weighed quantity with a mixture of salt and nitrate of potassium in a platinum vessel, producing sulphate of potassium, which, on the addition of a salt of barium, is precipitated as sulphate of barium. Care must be taken to perform the operation over a flame free from the vapour cf sulphur com pounds, which may vitiate the result by apparently increas ing the amount of sulphur present. For this reason, the flame of a spirit lamp is to be preferred in making the fusion to that of coal gas, which is rarely free from sulphur coin- pounds. Sulphur existing in the form of gypsum or sul phate of calcium may be removed by washing a sample with boiling water, and determining the sulphuric acid in the solution. The washed sample is then fused with nitre in the usual way to determine the proportion of sulphur existing as iron pyrites. This distinction is of importance in the examination of coals intended for iron smelting, as the sulphates of the earthy metals are reduced by the gases of the furnace to sulphides, which pass into the slag without affecting the quality of the iron produced, while the sulphur of the metallic sulphides in the ash acts pre judicially upon the metal. The difference between the original weight of the sample and that of the carbon, hydrogen, sulphur, and ash, after making allowance for hygroscopic water, is attributed to oxygen and nitrogen, which are not directly deter mined. The character of the ash affords some guide to the quality of the coal from which it is derived. Thus, a red tint is generally indicative of the presence cf iron pyrite s, and a light or white colour of its absence. Phosphorus if present will be found in the ash, and may be determined by the ordinary processes of analysis. A useful approximate method of determining the character of a coal is by ex posing a coarsely powdered sample of known weight, in a covered crucible, to a strong red heat as long as inflammable vapours are given off, when it is cooled and weighed. The loss of weight represents the volatile con stituents hydrogen, oxygen, and hydrocarbon gases, pro duced by destructive distillation, while the residual coke includes the ash, and is called fixed carbon. The character of the button of coke obtained is a good indication as to the caking or non-caking quality of the coal from which it is derived, and the amount of ash may be determined by burning it in a muffle or over the flame of a Bunsen burner, The fitness of a coal for gas -making is usually determined by operating upon a sample of a few pounds weight in a special apparatus which reproduces the pro- Besses of manufacture upon a small scale. One of the most important factors in the economic valuation of a coal, is the so-called calorific power or value, by which is usually understood the number of pounds of water at boiling point that can be evaporated by the complete combustion of one pound of coal. This may be obtained theoretically, when the composition of the coal is known, by computing the heating effect of the carbon and the disposable hydrogen ; but in the absence of an analysis, it may also be determined directly by several approximate methods. One of the most con venient instruments for this purpose is Thompson s calorimeter. This consists of a copper cylinder in which a weighed quantity of coal intimately mixed with chlorate or nitrate of potassium is deflagrated under a copper case like a diving-bell, placed at the bottom of a deep glass jar filled with a known weight of water. The gases produced by the combustion rising through the water are cooled, with a corresponding increase of temperature in the latter, so that the difference between the temperature observed before and after the experiment furnishes a mea sure of the evaporative power desired. The instrument is so constructed that 30 grains of coal are burnt in 29,010 grains of water, or in the proportion of 1 to 937, these numbers being selected that the observed rise of tempera ture iu Fahrenheit degrees corresponds to the required evaporative value in pounds, subject only to a correction for the amount of heat absorbed by ths mass of the instru ments, for which a special co-efficient is required, and must be experimentally determined. Another approximate method, due to Berthier, is based upon the reduction of oxide of lead by the carbon and hydrogen of the coal, the amount of lead reduced affording a measure of the oxygen expended, whence the heating power may be calculated, 1 part of pure carbon being capable of producing 34i times its weight of lead. The operation is performed by mixing the weighed sample with a large excess of litharge in a crucible, and exposing it to a bright red heat for a short time. After cooling, the crucible is broken and the reduced button of lead is cleaned and weighed. The re sults obtained by this method are less accurate with coals containing much disposable hydrogen and iron pyrites than with those approximating to anthracite, as the heat equivalent of the hydrogen in excess of that required to form water with the oxygen of the coal is calculated as carbon, while it is really about four times as great. Sulphur in iron pyrites also acts as a reducing agent upon litharge, and increases the apparent effect in a similar manner. The theoretical evaporative power of a coal found by either of the above methods is always considerably above that obtained by actual combustion under a steam boiler, as in the latter case numerous sources of loss, such as imperfect combustion of gases, loss of unburnt coal in cinders, &c., come into play, which cannot be allowed for in theoretical experiments. It is usual, therefore, to determine the value of a coal by the combustion of a weighed quantity in the furnace of a standard boiler, and measuring the amount of water evaporated by the heat developed. Various investi gations of this kind have been made at different times, both in Europe and America, the most extensive being the following : Johnson, Report on American Coals, Washington, 1844 ; De la Beche and Playfair, Three Reports on Coal suited to the Steam Navy, London, 1848-49-51 ; P. W. Brix, On the Heating Power of Fuel used in Prussia, Berlin, 1853 ; Hartig, Heating Power of Saxon Coal, Dresden, 1860. Tne following table of the average results obtained from these investigations shows the number of pounds of water evaporated for every pound of the different kinds of coal

burnt.