842 Dr. S. \V. J. Smith on the 



arrangement used it took a considerable time to collect a few 

 cubic millimetres. It was possible, however, to show that it 

 underwent contraction on explosion with oxygen in the 

 following way. A bubble of the gas, 4 mms. long, was 

 collected at the top of a tube which ended in a capillary 

 through which a fine platinum wire had been sealed. About 

 0*8 mm. o£ oxygen (prepared electrolytically) was added and 

 a second fine wire was pushed into the collecting tube from 

 below until it reached almost to the first. A spark was then 

 passed, and the remaining gas was found to occupy about 

 2*5 mm. of the tube*. Thus the gas evolved behaved like 

 hydrogen, the only constituent common to the two acids 

 employed. 



§ 12. Secondary effects with Sulphuric Acid. — Although, in 

 the absence of oxygen, the simplest direct interaction between 

 mercury and sulphuric acid is 



H g2 + H 2 S0 4 = Hg,SO« + H 2 , . . . (I.) 



there are other possible interactions of which the next f in 

 simplicity would be 



4Hg 2 + 5H 2 S0 4 = 4Hg 2 S0 4 + H 2 S +4H 2 0. . (II.) 



Here every fifth molecule of the acid may be supposed to 

 be reduced by the hydrogen resulting from the direct action 

 between four molecules of the acid and mercury. 



According to the view adopted in this paper the reaction I. 

 can go on only so long as the concentration of mercury salt 

 in solution does not exceed the value given by (i. a) ; but 

 according to the same view the reaction II. can occur before 

 and after this limit to reaction I. is passed. It probably does 

 not occur to the exclusion of I. because it involves a greater 

 rearrangement of the constituents of the reacting molecules 

 than is involved in the displacement of hydrogen (cf. Thom- 

 sen, I. c. p. 354 et passim) . 



To take the case of concentrated sulphuric acid, which was 

 carefully examined. The H 2 S of reaction. II. will interact 

 with a further quantity of the acid precipitating sulphur. It 

 will also in part precipitate the very nearly insoluble 



* Mr. W. F. Higgins kindly attempted to make a spectroscopic test 

 of the gas, but various difficulties were encountered which it did not 

 seem profitable to attempt to overcome since the gas had already been 

 proved to be neither II 2 S nor S0 2 nor oxygen. 



t In the reaction representing the secondary reducing effect of 

 hydrogen, viz., 



Hg a 4- (1 -f- a)H 2 S0 4 = Hg 3 S0 4 + (H 2 + aII 2 S0 4 ) 



the minimum value of a is 1/4. 



