Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/474

462 CHEMISTRY [HISTORY. air." It is air that is generated by fermentation, and that contributes to the briskness of Pyrmont and other mineral waters; 108 cubic inches of air are procured from a cubic inch of iron filings and the same quantity of oil of vitriol ; and 33 cubic inches of air are the result of distilling a cubic inch of dog s blood. Hales determined also the volume of air to be obtained by distilling certain quantities of amber, chalk, coal, grey pyrites, aqua-fortis, antimony, tobacco, and other materials, but apparently with no other end in view than the establishment of the fact that air is contained in a great number of substances. He had learned to interrogate, but not to cross-examine nature. The first important step towards a knowledge of the specific properties of the various gaseous bodies was that made in the middle of the 18th century by Dr Black, who experimentally proved that the causticity acquired on ignition by mild magnesia and lime was attributable not to the entrance into them of ponderable caloric, but to the expulsion of a peculiar kind of air, which occurred fixed, or in a state of combination, in the unburnt or mild earths, and caused them to be heavier before than after exposure to heat. He found it possible, in fact, to impart to these substances a large amount of heat, which became latent, whilst at the same time their weight was lessened by the loss of " fixed air " (carbon dioxide). It was discovered by Black that alkalies in contact with quicklime became caustic by giving up their fixed air to the lime, which was thereby increased in weight and rendered mild. It was thus, by employing the balance as an experimental test of the composition of bodies, that Black laid the foundation of quantitative chemistry, and in so doing gave the first occasion to the strife that twenty years later began to rage between the followers of Stahl and the antiphlogistians. Foremost in the number of those who after Black dis tinguished themselves as pneumatic chemists, was Dr J. Priestley (1733-1804), His first discovery, made in 1772, was nitric oxide gas, which he soon employed in the analysis of air. Boyle, more than a century before Priestley began his experiments, had stated in The Sceptical Chemist, that " without the addition of any extraneous body, quicksilver may by fire alone, and that in glass vessels, be deprived of its silver-like colour, and be turned into a red body ; from this red body, without addition, likewise may be obtained a mercury bright and specular as it was before." (J5oyZe s Works, ed. Birch, p. 352, Lond. 1744.) On the 1st of August, 1774, Priestley discovered that the red oxide of mercury evolved a gas when heated. This gas (oxgyen) being superior even to the air as a supporter of combustion was regarded by him as dephlogisticated air; the incom bustible part of the atmosphere he supposed to be saturated with phlogiston, on the assumption that a gas was so much the better adapted for supporting combustion as it contained within itself a smaller quantity of that body. Common air, by drawing phlogiston from burning substances, became, as he thought, phlogisticated air, and ou that account had no longer any attraction for phlogiston, or, in other words, any power of supporting combustion. The phlogiston evolved in the burning of combustibles and in the calcina tion of metals was supposed to unite with the atmosphere or the dephlogisticated air contained therein, and that which was produced by the action on the atmosphere of the phlogiston lost by the metals was the cause of the increased weight of their calces. The opinion that the air given off during the solution of metals in acid was their combustible constituent had been advanced in 1700 by Le"mery; and Priestley, guided apparently by the notion suggested by Cavendish s experiments, that to unite with acids metals must part with their phlogiston, considered inflammable air (hydrogen) either as identical with or at least very rich in that principle. In 1788, in order to explain the formation of water from a mixture of inflammable and dephlogisticated air, he put forward the hypothesis that water entered into the composition of these and of fixed and other airs ; inflam mable air, he thought, might be the principle of alkalinity, dephlogisticated air, as Lavoisier had shown, being the principle of acidity. To the last Priestley was an advocate of the phlogistic philosophy, and though unquestionably one of the fathers of modern chemistry was always, to quote the words of Cuvier, " un pere qui ne voulut jamais reconnoitre sa fille." Besides nitric oxide and nitrogen, Priestley first made known sulphurous acid gas, gaseous ammonia and hydrochloric acid, and carbon monoxide ; and he it was who, by showing that the condition of ammoniacal gas and of common air is altered by the transmission of electric sparks, led to Berthollet s analysis of ammonia, and Cavendish s discovery of the composition of nitric acid. Henry Cavendish (1731-1810), who, like Priestley, was of the phlogistic school, contributed by his discoveries and care fully conducted investigations, especially as regards gases, scarcely less than that experimenter to the advance in chemi cal knowledge which before the beginning of the 1 9th cen tury effected the subversion of the Stahlian philosophy. To him chemists are indebted for the invention of the pneumatic trough, and to him is due the first recognition of the importance of determining the specific gravities of the various gases. He established the radical difference between hydrogen and nitrogen, and discovered in 1781 that hydrogen and dephlogisticated air (oxygen), when ex ploded in a close vessel in proportions sufficient almost entirely to phlogisticate the burnt air, produced pure water ; and that water was also formed when a mixture of common air and inflammable air was exploded, a reduction of one- fifth of the bulk of the former air being then observable. According to Cavendish, water consisted of phlogiston and dephlogisticated air; inflammable air, of phlogiston and water ; the action of dephlogisticated upon inflammable air when exploded with it was to unite with its phlogiston to form water, and consequently to set free the water of the inflammable air ; thus both airs became water. (Trans. Roy. Soc., 1784 and 1785.) Lavoisier had shown in 1770 the incorrectness of the notion prevalent among chemists that water by continued boiling and redistillation could be transformed into an earth. Cavendish s discovery deprived it of the rank of an element, to which, according to the vague Aristotelian doctrines of the time, it was en titled, and thus prepared the way for the acceptation of correct and definite views concerning the elementary bodies. Lavoisier, availing himself of the facts ascertained by Cavendish, taught that oxygen, the so-called dephlogisticated air, was an element, and that combined with it was impon derable caloric ; inflammable air, or hydrogen, as he termed it, was another element, which had the power of disengaging from caloric a weight equal to its own of oxygen, with which it united to form water. The new doctrine did not, however, meet with very ready acceptance from the phlogistians. " It is inconceivable," writes one of them, " how water, which is absolutely incombustible, should have so combustible a body as inflammable gas is for one of its component parts ; whereas, by admitting pure air in its whole substance to be one of the component parts of water, and the other to consist of the base only of inflam mable gas, which being burnt by the passage of the electric spark through it, its phlogiston is converted into light and heat, the whole doctrine of the generation of water becomes plain and easy." (Hopson, Chemistry, 1789.) Among the most eminent of the contemporaries of Priestley and Cavendish that cherished a belief in the existence of phlogiston was the Swedish chemist Scheele

(1742-1786). In experiments made to ascertain the