266 Dr. Thomson on the [Oct. 



mined in this essay in every respect. Its atom is double the 

 specific gravity of its vapour, and it is a multiple of the atom of 

 hydrogen. 



V. Fluoboric Acid Gas. 



This acid gas Avas discovered by Gay-Lussac and Thenard. 

 The method of procuring it was much simplified by Dr. John 

 Davy. He showed that in order to obtain it, we have only to 

 mix together in a retort one part of finely pounded anhydrous 

 boracic acid, two parts of pounded fluor spar, and \2 parts of 

 sulphuric acid. When heat is apphed to this mixture, fluoboric 

 acid gas is extricated. It must be received over mercury. This 

 was the method employed to procure the fluoboric acid gas exa- 

 mined in my laboratory. Its specific gravity was found as 

 ollows : 



By first trial ■ 2-3694 



By second trial 2-3694 



The only person who examined the specific gravity of this 

 gas before us was Dr. John Davy : he found it 2-3709. These 

 two sets of experiments nearly agree ; the diff'erence between 

 them being only ^^-ih part. The nature of the constituents of 

 this gas is still too nirperfectly known to enable us to correct 

 our experimental results by calculation. But I thought it worth 

 while to record the result of our trials in this essay. They were 

 carefully made, and the time will come when they will have 

 their utility. 



In the preceding essay, I think I have established the true 

 specific gravities of 20 gases, deduced from them the real atomic 

 weights of eight simple bodies, and established two very import- 

 ant general laws. l.That the atomic weig,hts of the seven other 

 bodies are multiples of the atom of hydrogen ; and, 2. that the 

 weiglit of an atom of a gas is twice its specific gravity (or, in 

 some rare instances, four times its specific gravity) reckoning the 

 specific gravity of oxygen gas unity. 



It will probably be agreeable to the reader to see the true 

 specific gravities of these gases, and the specific gravities of 

 them which we determined by experiment. It will enable him 

 to judge of the degree of accuracy to which our experiments 

 were carried. 



P True specific gravities, Specific grav. from 



air being unity. experiment. 



Hydrogen 0-0694 0-0694 



Oxygen Mill MI17 



Azote 0-9722 .... 0-97286 



Chlorine 2-5000 2-5000 



Vapour of iodine 8-6805 8-68188 



Vapour of carbon 0-4166 0-41503 



