134 Mr. S. U. Pickering on the Densities 



was written, and the absence of any evidence (except Prof. 

 Sucker's recognition of the isolation of the tetrahydrate) that 

 my critics have looked at all into this aspect of the question, 

 must be my apology for again briefly stating my position. _ 



The properties originally studied by me were the densities 

 at four different temperatures (from which I calculated the 

 expansion by heat, and, in one case, the contraction on 

 mixing), the heat-capacity from to 10 per cent., and the 

 heat of dissolution. I also utilized Kohlrausch's experiments 

 on the electric conductivity. _ _ 



As it may be argued with some plausibility that the similar 

 form of the various density-curves would be likely to lead to 

 the same results when examined by a bent lath*, I have 

 contented myself with the following diagram, marking 

 (with dots) the positions of the changes between 2 per cent, 

 and 98 per cent.f shown by the densities at 8°, the contrac- 

 tions at 18°, and the three other properties above mentioned. 

 The working curves in these cases are very dissimilar ; yet 

 the closeness with which the dots are grouped around certain 

 percentages is most striking, and the absence of stray dots at 

 other percentages perhaps even more so. 

 Position of Breaks. 



20 40 60 80 100 



Per cent. H 2 S0 4 . 



H 2 S0 4 to 13 9 5-5 4 2 f 1 §. £ £ H a O 



It was not until after my examination of the curves was 

 near completion that I calculated the corresponding molecular 

 compositions^ . I then discovered that their percentages 

 corresponded to hydrates of very simple formulas, whose exact 

 percentages are marked by crosses on the diagram. Now, 

 considering that the various properties were quite independ- 

 ently examined, I ventured to think that the coincidences 

 afforded overwhelming evidence of some real changes in the 



* I do not myself attach much weight to this argument, as the similarity 

 is to a considerable extent superficial. The admission of all the density- 

 results would of course strengthen my argument. 



t The changes which occur at the extreme percentages cannot be 

 satisfactorily exhibited on a small scale, but they entirely confirm the 

 conclusions drawn from the others. 



% I was previously acquainted with the percentage composition of the 

 monohydrate only. 



