170 Dr. Thomson on the [Sept. 



gravity previously determined by Dr. Prout from other conside- 

 rations. When the results thus obtained in my laboratory are 

 contrasted with a flippant and petulant criticism on this very 

 specific gravity pitched upon by Dr. Prout, which appeared 

 some years ago in a London periodical publication, they cannot 

 but give birth to a train of reflections of rather a mortifying 

 nature. 



We may conclude then that the specific gravity of pure chlo- 

 rine gas is 2'5, or exactly 36 times heavier than hydrogen gas. 



Such are the results of the experiments made in my laboratory 

 to determine the specific gravity of the four simple gases. 

 Every possible attention was paid to insure the utmost degree of 

 accuracy of which my apparatus was capable ; and I am very 

 much mistaken if they do not fully confirm the specific gravities 

 previously determined by Dr. Prout from a most ingenious and 

 profound train of reasoning. Let us contrast the theoretical and 

 the experimental specific gravities of these four gases : 



Theory. Exper. 



Specific gravity of oxygen gas 1-11111 1-1117 



azotic gas 0-97222 0-9728 



hydrogen gas 0-06944 0-0694 



chlorine gas 2-50000 2-5000 



The greatest deviations are in the case of oxygen and azote ; 

 but even the greatest of these does not exceed -j-rVo-^h part, 

 which I am persuaded is within the hmits of unavoidable error. 

 Indeed I have some doubts whether we are in possession of the 

 means of obtaining azotic gas in a state of absolute purity, 



2. Let us now examine the specific gravity of the gaseous 

 compounds of the four simple gases. Here the general law 

 discovered by Gay-Lussac will be of great service to us by fur- 

 nishing us with a method of comparing our results with a correct 

 standard. My readers are aware that Gay-Lussac's general law 

 is this : " Gaseous bodies combine in certain very simple propor- 

 tions reckoning by volume : 1 volume of one gas unites with either 

 1 volume, or 2 volumes, or 3 volumes, or 1^ volume of another 

 gas. The new compound, if gaseous, has either exactly the 

 volume of the two constituent gases, or it loses a third, or the 

 half or three quarters of that volume." I have made a great 

 many experiments upon gaseous bodies with a view of satisfying 

 myself whether this general law be entitled to confidence. The 

 result has been the fullest conviction of its precision. I consider 

 it as one of the most important general laws hitherto discovered 

 in chemistry. Indeed 1 am of opinion that posterity will place 

 it as the most important of all the discoveries for which we are 

 indebted to Gay-Lussac, and that they will rank it among the 

 most important discoveries of the present age. 



1. We have two gaseous compounds of oxygen and azote; 

 namely, 1, protoxide of azote, the gas which Davy distinguished 



