262 Dr. Thomson on the [Oct. 



By first trial 0-9040 



By second trial 0'9025 



Mean 0-90325 



The only previous determination of the specific gravity of this 

 gas, as far as i know, is the one which I gave in my paper On 

 Phosphuretted Hydrogen already alluded to. 1 stated its speci- 

 fic gravity to be 0-90-22. Om- new determination differs only 

 .g^th part from the preceding one. This difference is but httle 

 when v*e take into consideration the peculiar difficulties which 

 attend the taking of the specific gravity of this gas. Unless the 

 flask be very completely exhausted indeed of common air, com- 

 bustion takes place when the phosphuretted hydrogen gas is let 

 into it. 'i'iiis combustion, by the deposition of phosphoric acid 

 which it occasions, has a tendency to increase the specific gravity 

 of the phosphuretted hydrogen gas beyond the truth. 0*9022 

 was not the average of the six results which I obtained from an 

 equal number of experiments — it was the experiment which 

 appeared to me the most successful. 



But we have a method of determining the specific gravity of 

 this gas which appears to me perfectly satisfactory. When 

 phosphorus is heated in hydrogen gas, a portion of the hydrogen 

 gas is changed into phosphuretted hydrogen, but no alteration 

 takes place in the volume of the hydrogen. It follows from this 

 that phosphuretted hydrogen is composed of 



1 volume hydrooen o-as "J j , . , , 



1 ,1 -^ c \ \ /- condensed into one volume. 



1 volume vapour or phosphorus J 



This is the same thing as to say that it is a compound of one 

 atom of hydrogen and one atom of phosphorus. Now we have 

 found the atoms of all the other bodies examined ; namely, 

 azote, oxygen, chlorine, carbon, and sulphur, to be multiples of 

 the weight of an atom of hydrogen. We have found hkewise 

 that the atomic weight of gaseous bodies is double their specific 

 gravity, provided we reckon the specific gravity of oxygen unity, 

 iet us apply these properties to the discovery of the weio-ht of 

 an atom of phosphorus, and of the specific gravity of phospho- 

 rutted hydrogen gas. 



I found the sp. gr. of this gas 0-9022 



The specific gravity of hydrogen is . . 0-0694 



0-8328 

 We see from this that phosphuretted hydrogen is composed of 



Hydrogen 694 or 1 



Phosphorus 8328 12 



