344 HISTORY OF SCIENCE. 



in short, he found himself engaged in researches which were destined 

 to shed a light on some very important points in chemistry. About this 

 time (1754) two medical professors at Edinburgh were engaged in a 

 lively discussion as to whether lime-water made with ordinary lime from 

 limestone, or that made from lime prepared by calcining oyster-shells, 

 were the more efficacious as a medicine. Black was an auditor of 

 these debates, and he determined to make experiments himself, with 

 a view of rinding in what manner the various alkaline earths differed 

 from each other. 



Black began by preparing magnesia (i.e., magnesium carbonate] by 

 mixing a solution of Epsom salts with one of potashes (potassium cat- 

 bonate), when a white powder falls, which, when washed and dried, 

 was distinguished by Black from lime by the presence of the follow- 

 ing characters : It effervesces with acids ; it forms with them com- 

 pounds quite different from those which lime produces with the same 

 acids; it precipitates lime from its combination with the acids; when 

 strongly heated, it does not produce quicklime, but a substance in- 

 soluble in water. In this last experiment Black's attention was drawn 

 to some remarkable differences between the common magnesia (car- 

 bonate) and a product obtained by its calcination (magnesium oxide). 

 The calcination produced a remarkable reduction in the bulk of the 

 white powder, and a still more remarkable diminution of its weight. 

 for every twelve parts were reduced to five. After calcination, too, it 

 dissolved in acids without effervescence, although the salts that it 

 formed with the several acids were not different from those formed by 

 the uncalcined magnesia. Black now asked himself this question : 

 What is the substance which is removed by the action of heat, and 

 the loss of which causes the diminution in the bulk and weight of the 

 magnesia? In order to find an answer to this query, Black put a 

 weighed quantity of magnesia (carbonate) into a glass retort, which was 

 connected with a receiver surrounded by cold water, in order to con- 

 dense into a liquid any vapours that might be driven off by the heat. 

 He heated his retort to redness, and in the receiver he obtained only 

 a, " very small quantity of watery fluid," yet the magnesia had lost the 

 greater part of its weight. He was astonished at this, and Hales' fa- 

 vourite theory suggested itself to his mind, namely, that air had been 

 expelled from the magnesia by heat, and this appeared to be confirmed 

 by the fact of acids expelling air (carbonic acid gas) from the uncalcined 

 magnesia. In order to test this theory, he proposed to restore, if pos- 

 sible, to the calcined magnesia the air it had lost. In thinking how 

 this was to be done, he first asked himself: From whence did the 

 magnesia originally receive the air? It was expelled when the mag- 

 nesia combined with sulphuric acid, so that it did not exist in the 

 Epsom salts, and therefore it must have received this air from the alkali 

 (carbonate of potash) which was used to precipitate it. 



Very notable is Black's next experiment, because it shows that 



