I SO Dr. Thomas Thomson s Observations 



was any difference between the air of London and the 

 country, by filling bottles with air on the same day, and 

 nearly at the same hour, at Marlborough-street, and at 

 Kensington ; but the difference between them was never 

 more than might proceed from the error of the experiment. 

 And, by taking a mean of all, there did not appear to be 

 any difference between them. 



These important experiments and conclusions of Mr. 

 Cavendish were forgotten or disregarded by chemists, till 

 nearly the beginning of the present century, when Ber- 

 thollet announced, that he had examined the atmospherical 

 air in Egypt, and bad always found it composed of about 

 79 volumes of azotic, and 21 volumes of oxygen gas. Sir 

 Humphrey Davy made the same observations somewhat 

 later, of the atmosphere in the neighbourhood of Bristol, 

 and from the coast of Guinea; and in the year 1801, I 

 ascertained, that the composition of the air, at Edinburgh, 

 ■was precisely similar in its constitution. In consequence 

 of the knowledge of these and some other facts of the same 

 kind, chemists unanimously adopted the conclusion of 

 Cavendish, that the composition of common air is constant. 

 Hence, the inference, that it is a chemical compound of 

 oxygen and azote is unavoidable. 



In 1803, an elaborate set of experiments was published 

 by Humboldt and Gay Lussac {Ann. de Chimie., liii. 251), 

 on the method of analyzing mixtures of oxygen and inflam- 

 mable gases, by means of Volta's eudiometer, and, among 

 other conclusions, they affirmed, that common air (abstract- 

 ing its impurities) is composed of 79 volumes of azotic and 

 21 volumes of oxygen gases; the very same proportions 

 which had been already adopted by Berthollet and Davy. 



In the year 1808, Gay Lussac read a paper to the Society 

 of Arcueil, which was published in the second volume of 

 the Memoirs of that Society, proving, that gases always 

 combine either in equal volumes, or one volume of the one 

 with two volumes, or with three volumes, or with four 

 volumes of the other, and never in any other ratios. This 

 important conclusion was called in question by Mr. Dalton ; 

 but has been acquiesced in by all other chemists, and has 

 for many years been adopted as a fundamental principle in 

 chemistrv. 



