22 Proceedings of the Royal Physical Society. 



discovery of latent heat was certainly Black's greatest work ; 

 but that which has, perhaps, most interest for chemists, was 

 his researches on the fixed alkalies, the results of which were 

 published in 1755, and which rendered signal service in con- 

 tributing in no small degree to the final overthrow of the 

 phlogistic theory. Black brought a great power to bear on 

 scientific research when he introduced the use of the balance 

 so extensively in his investigations, and, indeed, it was by 

 the skilful use of this invaluable instrument that he was 

 enabled to complete satisfactorily some of his most famous 

 researches. Thus, before Black's time, the carbonated or 

 mild alkali, as it was then called, had always been regarded 

 as a substance of a simpler composition than the caustic 

 alkali. The latter, instead of being looked upon as something 

 less than the former, was regarded as a compound of the 

 former — that is, the carbonated alkali — with what was desig- 

 nated, somewhat vaguely it must be admitted, the principle 

 of combustibility. So that when ordinary limestone, or what 

 we call carbonate of lime, was exposed to the heat of a 

 furnace, it was supposed to unite with the principle of com- 

 bustibility, and thereby to become more complex in its 

 nature than was the original limestone. And further, if this 

 quicklime, resulting from the heating operation, was boiled 

 with a solution of carbonate of potash, the change which took 

 place was explained by saying that the carbonate of potash 

 became caustic by taking from the quicklime the principle 

 of combustibility which the latter had obtained from the 

 fire. Black showed, however, by means of the balance, that 

 the heating of magnesia alba, which rendered it no longer 

 capable of effervescing with acids, was accompanied by 

 a loss in weight, which certainly would not have been the 

 case had it taken up any principle of combustibility from 

 the fire. Black crave us, indeed, the correct information we 

 now possess regarding the carbon ating and decarbonating of 

 alkalies. He showed most distinctly that carbonic acid, or, 

 as he called it, fixed air, is given off from carbonated alkalies 

 by certain modes of treatment, and that the change in their 

 properties which these substances suffer when thus treated, 

 and so rendered caustic, is owing to this loss, and not to the 



