146 GREAT MEN OF SCIENCE 



families in England ; he lived almost continually in London, 

 in solitude and great retirement, devoted entirely to science. 



Cavendish discovered in 1766, that is before Scheele's 

 and Priestley's experiments on gases, hydrogen gas as a 

 peculiar body entirely different from ordinary air. It was 

 the second, after the carbonic acid already fully investigated 

 by Black, of the kinds of gas recognised as fundamentally 

 different from atmospheric air. It had of course long been 

 known that iron dissolves in acids with effervescence, and 

 Boyle already noticed the inflammability of the kind of air 

 which was emitted, but without investigating it further. 

 This investigation was carried out with great thoroughness 

 by Cavendish. He noticed, and also was the first to deter- 

 mine, its strikingly small specific gravity, whereby he also 

 took temperature and pressure into account.^ He measured 

 the quantities of gas developed by the solution of given 

 weights of iron, zinc, and tin in different acids, investigated 

 the explosive mixtures of hydrogen and air, and made many 

 other contributions to our knowledge of this gas. 



He also investigated later the gaseous products of combus- 

 tion of different substances, and found that carbonic acid is 

 only produced in the combustion of vegetable and animal 

 substances. 



In the year 1773 he carried out electrical investigations, 

 which led him to the law of inverse squares for electric force, 

 and even to an acquaintance with what we call to-day the 

 dielectric constant, but he left all this unpublished. ^ 



^ Black soon afterwards pointed out that thin vessels filled with this gas 

 would rise in the air. Seventeen years later, this was put into practice on 

 a large scale by Charles, professor of Physics in Paris, and thus our 

 present-day airships originated. 



2 See Maxwell, Papers II, page 612, and Treatise on Electricity and 

 Magnetism, 1892, vol. i, pages 80 ff. These are investigations which 

 were taken up more than ten years later by Coulomb, and in other matters 

 sixty years later by Faraday, and carried further. Scarcely anything 

 else, apart from his published successes, could give us a higher opinion 

 of Cavendish as a scientist, than this pioneer work, combined with his 

 holding back results which obviously did not completely satisfy him. 



