THE GREAT LONDON CHEMISTS 35 



substances. He also was acquainted with what Black 

 termed ' latent heat/ that is, the heat absorbed during the 

 evaporation of liquids, or which is evolved during the 

 condensation of gases or vapours, or the solidification of 

 liquids. 



As this essay deals with Cavendish as a chemist, I shall 

 treat very shortly of his physical work. One of the most 

 important of his investigations has reference to the 

 cause of the shock given by that curious fish the 

 torpedo. By constructing a species of artificial torpedo, 

 he proved that the shock was due to an electric discharge ; 

 and what is more, he was the first to distinguish between 

 electric quantity and electric intensity. Indeed, these 

 terms are due to him, as Faraday has acknowledged. 



In 1783, 1786, and 1788, he published three papers on 

 freezing, in which his views on the nature of heat were 

 expounded. The first of these deals with the freezing of 

 mercury ; the second and third, with the congelation of 

 the mineral acids, and of alcohol. He objected to Black's 

 expression, ' the evolution, or setting free of latent heat/ 

 as involving an hypothesis that the heat of bodies is 

 owing to their containing more or less of a substance 

 called the matter of heat. He preferred to adopt Boyle's 

 and Sir Isaac Newton's supposition that heat consists in 

 the internal motion of the particles of bodies. And he 

 therefore uses the expression ' heat is generated.' 



An interesting part of the last of these papers is a 

 passage in which he anticipates Richter's tables of the 

 equivalents of the acids and bases, not by any elaborate 

 disquisition, but as a device for estimating the strength of 

 sulphuric acid. In 1788 he wrote : ' The method I used 

 was to find the weight of the plumbum vitriolatum formed 

 by the addition of sugar of lead, and from thence to com- 

 pute the strength, on the supposition that a quantity of 

 oil of vitriol, sufficient to produce 100 parts of plumbum 



