66 Britain's Heritage of Science 



it gave out again when brought into contact with water. 

 The corresponding compound of magnesia behaved similarly, 

 and was not clearly distinguished from the calcium salt. 

 Magnesia had then already some importance as a drug, 

 and the title of Black's dissertation " De humoro acido a 

 cibis orto et magnesia alba " indicates that it was the medi- 

 cal aspect that led him to the research. Black proved that 

 the current explanation was wrong, and that, instead of 

 absorbing anything, limestone, on heating, lost in weight, 

 and gave out a gas, which he collected and identified with 

 Helmont's " gas sylvestre." He definitely proved that this 

 gas, now known as carbonic acid, differed from air, because 

 it could combine with caustic soda and potash, which air 

 could not; he also showed that atmospheric air always 

 contained small quantities of it. Black further established 

 the essential differences between the behaviour of calcium 

 and magnesium compounds. His use of the balance in 

 these researches justifies the claim that has been made on 

 his behalf of being the father of quantitative chemistry. 



In his researches on heat, Black showed an equal power 

 of selecting the fundamentally important questions, and of 

 treating them with experimental skill and scientific precision. 

 His results were explained in his lectures, but many of them 

 remained unpublished until after his death It is, therefore, 

 not always easy to fix the dates at which his discoveries 

 were communicated to his students, so as to compare them 

 with similar results arrived at in other countries, notably 

 by Wilcke at Stockholm, and Deluc, who, born in 1727 at 

 Geneva, left his native town at the age of forty-three and 

 after various travels settled down in England, and died at 

 Windsor in 1817. There is no doubt, however, that Black 

 was the discoverer of latent heat. Deluc had noted the slow 

 melting of ice, and made the observation that when a mixture 

 of ice and water is heated, the temperature of the water 

 remains constant until all the ice is melted, but Black went 

 a good deal further, and not only measured the heat required 

 to melt the ice, but showed it to be the same in amount as 

 that which was set free in freezing the water. He applied 

 the term "latent heat," which is still in use, and his 

 measurements were correct to two per cent. The corre- 



