1815.] 



the Hon, Henry Cavendish, 



11 



gas. This discovery induced him to e'lpploy nitrons gas as a test 

 of the quantity of oxygen present in common air ; and various 

 instruments were contrived to facilitate the mixture of the gases, 

 and the measurement of the condensation. As the goodness of 

 air, or its fitness to support combustion, and maintain animal 

 life, was conceived to depend upon the proportion of oxygen 

 gas which it contained, these instruments were distinguished by 

 the name of eudiometers. The best of them vvas contrived by 

 the Abbe Fontana, and is usually distinguished by the name of 

 the Eudiometer of Fontana, Philosophers, in examining air by 

 means of this instrument, at various seasons, and in various 

 places, had found considerable dilFerences in the diminution of 

 bulks. Hence they inferred that the proportion of oxygen varied^ 

 and to this variation they ascribed the healthiness or noxiousness 

 of particular places. Mr. Cavendish examined this important 

 point with his usual patient industry and acute discernment. He 

 ascertained that the apparent variations were owing to inaccu- 

 racies in making the experiment ; and that, when the requisite 

 precautions were taken, the proportion of oxygen in air was 

 found constant in all places and at all seasons. He determinedj 

 also, by a correct experiment, that air is a mixture of very nearly 

 21 parts by bulk of oxygen gas, and 7^ parts of azotic gas, 

 (Phil. Trans. 1783, vol. Ixxiii, p. 106.) 



For many years it was believed by philosophers that mercury 

 was essentially liquid, and that no degree of cold was capable of 

 congealing it. Professor Braun's accidental discovery, that it is 

 frozen by cold like other liquids, was at first doubted ; and when 

 it was finally established by irrefragable experiments, it was 

 concluded, from the observations of the Petersburgh philoso- 

 phers, that its freezing point was not less than several hundred 

 degrees below zero. It became an object of great importance 

 to determine the exact point of the congelation of this metal by 

 accurate experiments. This was done at Hudson's Bay by Mr. 

 Hutchins, who followed a set of directions given him by Mr. 

 Cavendish. From these experiments Mr. Cavendish deduced 

 that the freezing point of mercury is very nearly 39° below zero 

 of Fahrenheit's scale. (Phil. Trans. 1783, vol. Ixxiii. p. 303.) 



These experiments naturally drev/ the attention of Mr. 

 Cavendish to the phenomena of freezing, to the action of 

 freezing mixtures, and the congelation of the acids. He 

 employed Mr. M'Nab, who was settled in the neighbourhood 

 of Hudson's Bay, to make requisite experiments ; and he pub- 

 lished very curious and important papers on these subjects, (Phil. 

 Trans. 1786, vol. Ixxvi. p. 241 ; and I788, vol. Ixxviii. p. 1G6.) 

 He explained the phenomena of congelation exactly according 

 to the theory of Dr. Black, rejecting only the hypothesis that 

 beat is owing to the presence of a peculiar matter^ and thinking 



