434 POPULAR SCIENCE MONTHLY. 



the shadow of the unseen world should enshroud him in its darkness. 

 After an illness of only three days, the only one he ever had, he called 

 his servant, told him he was going to die, and commanded him to stay 

 away and to keep everyone else away until the event was over. The 

 servant obeyed, and, when he returned. Cavendish had breathed his 

 last. 



An intellectual head thinking, a pair of wonderfully acute eyes 

 observing, and a pair of very skilful hands experimenting and record- 

 ing, are all that we realize in reading his memorials. His theory of 

 tlie universe seems to have been that it consisted of a multitude of ob- 

 jects to be weighed, numbered and measured; and the vocation to 

 which he thought himself called was to weigh, number and measure as 

 many of these objects as his threescore years would allow. Whenever 

 we catch sight of him, we find him with his measuring rod and balance, 

 his graduated jar, thermometer, barometer and table of logarithms. 

 Most of his researches were avowedly quantitative; he weighed the 

 earth, he analyzed the air, he discovered the compound nature of 

 water, and he noted with numerical precision the actions of the ancient 

 element fire. Everything pertaining to each, to which a quantitative 

 value could be attached, was set down in figures, before it went out to 

 the scientific world with its passport signed and sealed. In all his re- 

 searches he displayed the greatest caution, not from hesitation or 

 timidity, but from his recognition of the difficulties which attend the 

 investigation of nature. Cavendo tutus was the motto of his family, 

 and seems ever to have been before him, so that he well deserves the 

 title — 'Father of Quantitative Physics.' 



His first recorded scientific work was 'Experiments on Arsenic' 

 (1764). In 1765 'Experiments in Heat' were performed which, though 

 written out for a friend, were not made public till nineteen years 

 later, but which, had they been published in 1764, would have given 

 Cavendish precedence to Black in some of his discoveries as to 'latent 

 heat' and 'specific heat,' and equal merit in others. In his first public 

 contribution to science, 'Experiments on Factitious Airs,' sent to the 

 Royal Society in 1766, he defines 'factitious air' as air which is driven 

 off when compounds are heated or treated with acids, and the ques- 

 tions of the permanent elasticity of 'factitious airs,' their solubility in 

 different liquids, their power to support combustion, their specific 

 gravity, and likewise their combining equivalents, were all carefully 

 considered. 'Fixed air' (COo) was only a particular kind of factitious 

 air driven off from the alkalis (carbonates), and he found that when 

 it was mixed with common air in the proportion of one part to 

 nine, it rendered the air unfit for respiration. Cavendish first isolated 

 and experimented with hydrogen, though he cannot be called its dis- 

 coverer, for Paracelsus, about 1540, obtained it by acting on metals 



