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104 BLACK. his mind in 1752,and hecontinuedhis observations till 1754, when he published "Dissertatio Inauguralis de Humore Acido a Cibis Orto et Magnesia Alba." His observations and reasonings on the subject, were more fully developed in " Experiments upon Magnesia Alba, Quicklime, and some other Alkaline Substances," which were read before the Literary Society of Edinburgh, and afterwards inserted in the second volume of the Essays Physical and Literary, published by that Society. The experiments in this paper are simple, but. inge- niously devised; it is concise, yet perspicuous ; and the deductions are so just, that it is considered a most ex- cellent model of composition, reasoning, and arrangement. The facts it develops are now so well known to every one as forming one of the first elements of chemistry, that it would be unnecessary to detail them, were it not to exem- plify the bistory of the scieuce as far as it relates to the discovery of that immense class of substances known by the name of gasses Magnesia had hitherto been confounded with the other absorbent earths, being conceived to be merely a modii- cation of lime. The experiments of Dr. Black proved, that it was distinet from that substance; and he then pro- ceeded to investigate its affinity to acids. In endeavouring to convert magnesia into quicklime by fire, he discovered that a subtile part was extricated in the form of air, which had been imprisoned under a solid form. i This accounted for the effervescence of magnesia with acids before, but not after, calcination. Calcined magnesia, by a very happily-conceived experiment, was discovered to absorb from common vegetable alkali (potass) as much air as it bad lost by exposure to fire ; thus the same air which was contained in magnesia, was detected in alkali, and in limestone unburnt. From these discoveries the author acately concluded, that the cause of the causticity of quicklime was the separation of the above air by fire from calcareous earth; and that lime