204 Dr. Thomson's Reply to M. Vauquelin. [Sept. 



derived my knowledge of the properties of bihydroguret of phos- 

 phorus. 



Davy found that the bulk of this gas was doubled when 

 potassium was heated in it, or when sulphur was sublimed in it. 

 He states that three volumes of it require for complete combus- 

 tion more than five volumes of oxygen gas ; and that it is a 

 compound of one part by weight of hydrogen and five parts of 

 phosphorus. Now this (when the requisite corrections are 

 made) is the same as if he had said that it is a compound of two 

 atoms hydrogen and one atom phosphorus. In consequence of 

 these statements of Davy, I thought myself entitled to conclude 

 that the gas which he described was a compound of two atoms 

 hydrogen and one atom phosphorus, and I called it in conse- 

 quence bihydroguret of phosphorus. 



I had myself determined by experiment that when phosphu- 

 retted hydrogen gas is left for some time over water or mercury, 

 it deposits phosphorus without any perceptible alteration in its 

 bulk, loses the property of burning spontaneously when mixed 

 with atmospheric air, and yet still continues a compound of 

 phosphorus and hydrogen. Hence I inferred that it had become 

 the identical gas described by Davy. But I made no experi- 

 ments on the effect produced on it by potassium and sulphur, 

 relying on the accuracy of Davy's statements. 



But I still considered that it would be necessary to determine 

 the point by direct experiments ; and more than two years have 

 elapsed since I set about examining the subject. I left a quan- 

 tity of pure phosphuretted hydrogen in a graduated glass jar 

 over mercury for six months ; namely, from January to August. 

 The mercurial trough was placed nearly in the middle of my 

 laboratory, which is a large room, and so that the sun never 

 shone on the gas. Another jar filled with the same gas was 

 placed over mercury in the dark, and left for the same length of 

 time. But it must have been accidentally overturned, and again 

 replaced by some person ; for when I examined the gas, I found 

 it to be common air. The gas standing in the middle of the 

 laboratory had not sensibly altered its bulk ; but a portion of 

 phosphorus had been deposited on the inside of the jar. It did 

 not burn when mixed with common air or oxygen gas ; but 

 still had the peculiar smell which characterizes phosphuretted 

 hydrogen gas. Its bulk was not in the least altered by sublim- 

 ing sulphur in it, so that in this respect (as Vauquelin states) it 

 resembles phosphuretted hydrogen, and differs from Davy's gas. 

 One volume of it required for complete combustion 1^ volume of 

 oxygen gas. When a volume of it is mixed with 075 volume of 

 oxygen, and an electric spark is passed through the mixture, 

 detonation takes place, and the oxygen disappears ; but the resi- 

 dual gas is within one-seventh of a volume, and on adding 

 another half volume of oxygen gas, it may be detonated again, 



