On the Atomic Weights of Bodies. 179 



Article II. 



Observations on the Atomic Weights of Bodies. By Thomas 

 Thomson, M. D., F. R.S. L. & E., Regius Professor of 

 Chemistry in the University of Glasgow. 



When we attempt to establish the atomic weights of bodies 

 with precision, it is obvious, that the first step must be an 

 accurate knowledge of the specific gravity of oxygen gas, 

 because an error in it will affect the whole of our subsequent 

 determinations, and prevent us from perceiving the beauti- 

 ful simplicity which nature has followed in establishing 

 these atomic quantities. Atmospheric air, it is well known, 

 consists, in a great measure, of oxygen and azotic gases ; 

 which, as was first shewn by Mr. Cavendish, exist always 

 in the atmosphere in the very same proportions. But there 

 are constantly present in it carbonic acid gas, and the 

 vapour of water ; the amount of both of which, especially 

 of the latter, varies considerably. Unless, therefore, we 

 take the precaution to free common air from all admixture 

 of carbonic acid and vapour before we attempt to analyze 

 it, we cannot expect to obtain results which agree ac- 

 curately with each other. I suspect that few experimenters, 

 who turned their attention to the constitution of atmos- 

 pherical air, have been at any trouble, either to dry it, or 

 to free it from carbonic acid gas, before subjecting it to 

 analysis. 



The first person who shewed that atmospherical air does 

 not vary in its constitution (abstracting the carbonic acid and 

 vapour which are constantly varying), was Mr. Cavendish, 

 (Phil. Trans., 1783, p. 106.) During the last half of the 

 year 1781 , he tried the air of near 60 different days, to find 

 whether the proportion of oxygen was sensibly greater at one 

 time than another ; but found no difference that he could be 

 sure of; though the wind and weather, on those days, were 

 very various : some of them being very clear and fair, 

 others yery wet, and others very foggy. He tried the air 

 at different times of the same day, without finding any 

 alterations in the proportions of its constituents. He also 

 made several trials with a view to determine whether there 



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