Results of Experiments. 425 



hydrates of sulphuric acid and other solutions (Journ. C. S. 

 March 1890; Phil. Mag. 1890, xxix. p. 433) , with special refer- 

 ence to the density, expansion, heat of combination, and electric 

 conductivity. While this work was in progress Prof. 0. J. 

 Lodge (' Nature/ July 18th, 1889) gave a word of warning to 

 experimentalists on the over-pressure of formulae, and Arrhe- 

 nius (Phil. Mag. xxviii. p. 37) severely criticised a preliminary 

 account of Mr. Pickering's methods, pointing out that his 

 equations for sulphuric acid required the introduction of 

 nearly sixty constants, from which anything could be proved. 



It is always an ungracious and difficult task to criticise 

 experiments upon which much time and care have been ex- 

 pended, and in this case the difficulty is increased by the 

 omission from Pickering's paper of many experimental details 

 and by the great number and variety of the experiments. To 

 minimize the latter the following remarks will be confined to 

 the density of solutions of sulphuric acid at 28° C, containing 

 from 50 to 38 per cent, of real acid (Table II.), Journ. C. S. 

 1890, p. 142. 



The objections may be conveniently divided into those 

 which impugn the extreme accuracy of the experimental 

 results, and those against the mathematical deductions. 



Experimental : — 



(1) Mr. Pickering's results do not agree with those of 

 Mendeleeff ; he says : — " MendeleefFs values by no means 

 warranted the conclusion that the first differential consisted of 



straight lines The curves required a second differentiation 



before they yielded a rectilinear figure " (Journ. C. S. 1890, 

 p. G6). 



(2) As the mean of eight volumetric determinations start- 

 ing from pure silver, the acid used was assumed to contain 

 99*85 per cent, of real acid. It is a little startling in the case 

 of experiments the whole object of which is extreme accuracy 

 to read for the separate values found 



B 



with a difference of *1 per cent, on each side of the mean used. 

 (3) The thermometer read to *1 millim. = , 004°C. and the 

 balance weighed to '0001 gram. The densities were deter- 

 mined in a " piknometer " holding 25 cubic centim., it was 

 apparently of the old form, and a glass plate was placed over 

 the open capillary. The objections to this form of instrument, 



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