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 recording, are all that we realize in reading his memorials. His theory of the universe seems to have been that it consisted of a multitude of objects 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 researches 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 questions 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' (CO2) 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 discoverer, for Paracelsus, about 1540, obtained it by acting on metals
