244 Dr. Thomson on the [Oct. 



A little consideration will satisfy the reader that when the 

 gaseous bodies of which these compounds are constituted unite 

 together they undergo no change whatever in their bulk. Pro- 

 bably, therefore, the force of affinity which occasions the union 

 is considerably weaker than the affinity by which the four pre- 

 ceding gaseous compounds are kept united. 



I shall proceed in the remaining part of this essay to endeavour 

 to establish the exact specific gravities of the gaseous com- 

 pounds formed by the union of the simple gases with the solid 

 bodies carbon, sulphur, and phosphorus. These specific gravi- 

 ties will be found of great importance, because they will put it in 

 our power to determine with precision the atomic weights of 

 carbon, sulphur, and phosphorus ; bodies which act so very pro- 

 minent a part in chemistry, and which enter into so vast a 

 number of important combinations. 



I. Carbon. 



Carbon forms gaseous compounds with oxygen, hydrogen, and 

 azotic gas, and there is a gaseous compound known composed 

 of carbon, oxygen, and chlorine- 

 Carbon and oxygen form . . carbonic acid, 



carbonic oxide. 

 Carbon and hydrogen form olefiant gas, 



carburetted hydrogen. 

 Carbon and azote form. . . cyanogen, 

 The triple compound is . . chlorocarbonic acid or phosgene gas.. 



1. Carbonic Acid Gas. — A quantity of very pure carbonic acid 

 gas was prepared by dissolving calcareous spar in muriatic acid. 

 Its specific gravity taken three times with the necessary precau- 

 tions to ensure accuracy w as as follows : 



By first trial 1-5266 



By second trial 1*5268 



By third trial 1-5268 



Mean = 1-52673 



The first person who attempted to determined the specific 

 gravity of this gas was Mr. Cavendish, who fixed it at 1-57 ;* 

 but his data were obviously inaccurate ; though we have not the 

 means of rectifying them. He filled a bladder capable of holding 

 100 ounce measures with the gas, and weighed it. The gas was 

 then squeezed out, and the bladder being weighed again was 

 found to weigh 34 grs. less than at first. Now if we suppose 

 100 ounce measures to be equal to 189-8 cubic inches, and 100 

 cubic inches of common air to weigh 30-5 grs. it is obvious that 

 Mr. Cavendish's experiment will give us the specific gravity of 



• Phil. Trans, 1766, p.l4U 



