1824,] Dr. Thomson's Reply to M. VaiUjite/in. 203 



the base, it appears probable that the blue variety of verdigris 

 does not possess so simple a constitution as is indicated by the 

 foregoing formula, but that it may be a compound of the first 

 salt with hydrated peroxide of copper and water of crystalliza- 

 tion ; on these grounds, its composition would, perhaps, be 



more accurately represented by the formula Cu A- + Cu Aq°- + 

 10 Aq, in the last term of which, the water of crystallization is 

 distinguished from the portion which acts as an acid when in 

 combination with the peroxide of copper. 



Article VII. 



Reply to M. Vauquelin' s Remark on a supposed Contradiction in 

 Dr. Thomson's System of Chemistry. 



(To the Editors of the Annals of Philosophy.) 



GENTLEMEN, Glasgoiv, Aug. 10, 1824. 



On my return home yesterday from Berwickshire, I found the 

 number of your journal for the present month lying on my table. 

 My attention was naturally attracted to the notice in p. 147 

 translated from the Ann. de Chim. and entitled " Note on a 

 Contradiction in Thomson's System of Chemistry respecting 

 Phosphuretted Hydrogen Gas. By M. Vauquelin." M. V. ob- 

 serves, that 1 state in the first place that phosphuretted hydrogen 

 contains its own volume of hydrogen united to a volume of 

 phosphorus vapour; that when' it is exposed to the direct rays 

 of the sun, a quantity of phosphorus is deposited, and bihydro- 

 guret of phosphorus obtained ; and that when sulphur is heated 

 in bihydroguret of phosphorus, the bulk is doubled, and two 

 volumes ofsulphuretted hydrogen obtained. M. Vauquelin then 

 goes on to show, that these two statements are inconsistent with 

 each other, and that when sulphur is heated in phosphuretted 

 hydrogen gas, only a very slight increase of bulk takes place, 

 lie mentions also that the deposition of phosphorus takes place 

 without any exposure to the sun's rays, and more rapidly during 

 the night and in a dark place than during the day. 



Had M. Vauquelin paid attention to the account whioh I have 

 given of bihydroguret of phosphorus in the passage of my 

 System of Chemistry which he quotes, he would have seen the 

 cause of the apparent contradiction which he notices. It is 

 owing to my having supposed that phosphuretted hydrogen gas, 

 when altered by keeping, is converted into the gas which Sir H, 

 Davy obtained' by heating hydrated phosphorous acid, and which 

 he described in Phil. Trans', for 1812, p, 408. To this account 

 1 referred in my System, thus pointing out the source whence t 



