ix SULPHUR AND PHOSPHORUS 169 



Dalton (New System, II. 389-390) also attempted to 

 determine the proportions in which the two gases combine, 

 by uniting them over mercury in presence of water or with 

 the help of electric sparks. In the former case the action 

 was incomplete at the end of twelve days ; in the latter 

 case he found that "the mercury becomes oxidised, and 

 consequently liable to form a union with either of the 

 acids." 



Gay-Lussac's analysis of sulphuric acid (1807). 

 A successful analysis of sulphuric acid was made by 

 Gay-Lussac in 1807 in the course of an investigation on 

 the " Decomposition of the Sulphates by Heat " (Mem. 

 Soc. <?Arcueil> I. 215 251). He found that many of the 

 sulphates could be decomposed ; some were decomposed 

 easily, giving as products the oxide of the metal and white 

 fumes of SULPHURIC ANHYDRIDE ; others, requiring a higher 

 temperature for decomposition, gave off a gaseous mixture 

 of sulphurous anhydride and oxygen, leaving behind as 

 before a residue of oxide or earth. The quantitative 

 analysis of sulphuric anhydride was effected by decomposing 

 alum, the gases from the decomposition being collected 

 over mercury and analysed by absorption with potash. 



Gay-Lussac writes : 



" The first sulphate that I heated was sulphate of copper. 

 It first liberated water ; but as soon as the retort was red-hot, 

 white vapours of sulphuric acid x were produced, which were 

 accompanied by a nebulous gas, smelling strongly of 

 sulphurous acid, and in which after being washed a match 

 inflamed several times. This gas was then a mixture of 

 sulphurous acid x gas and oxygen . . . The two gases were in 

 volume almost in the ratio 2 to i ; but I will return later to 

 the exact determination of this ratio, and the way in which 

 the sulphuric acid decomposes" (pp. 217 218). 



" Sulphate of iron undergoes the same decomposition by 



1 In this passage the names "sulphurous acid" and "sulphuric acid" 

 are used to describe the anhydrides. 



