242 Dr. E. J. Mills on Gladstone's Experiments 



of change per unit interval is proportional to the amount of sub- 

 stance then changing. 



As Gladstone's results were the first in which the continuity 

 of the chemical process was experimentally demonstrated (at any 

 rate on a sufficient scale), I felt much interested in ascertaining 

 whether Esson's equation would apply to them — especially when 

 I considered how few have been the contributions to chemical 

 dynamics, the laborious (and consequently unpopular) nature of 

 such researches, and the inexpediency of allowing good work to 

 remain dumb or unexpressed. 



II. The colorimetrical method, which was used throughout by 

 Gladstone, has considerable disadvantages, and is most service- 

 able when only small quantities, as in the case of the Nessler 

 test, have to be measured and an inaccuracy of about 5 per cent, 

 is of no consequence. It is probable that the observer's estimate 

 of colour varies during a long course of experiments, and is 

 really under training in the earlier ones; so that, as will 

 actually be found below, all the more serious errors occur, as a 

 rule, at the outset. We must also remember that colour-effects 

 in solutions are not unfrequently slow in attaining their maxi- 

 mum, thus making a particular observation too low; on the 

 other hand, the subsequent arrival of this maximum will make 

 a following observation too high : hence also, by virtue of com- 

 pensation, the later observations may be expected to be more 

 correct. 



A farther difficulty lies in the computation itself. The amount 

 of chemical energy (or substance) originally present is not given 

 in terms of the reagent, and has to be arrived at by successive 

 and wearisome approximations; and these might perhaps have 

 been carried a stage further with advantage. Again, the suc- 

 cessive values of x are very seldom given in the experiments, 

 which had not been arranged to test any particular hypothesis ; 

 they had consequently to be obtained by graphic interpolation 

 on curves which, for such a purpose, should have been consider- 

 ably longer. 



If we bear in mind these and other drawbacks, we shall regard 

 the coincidence between theory and experiment as very striking. 



IIT. Ferric Nitrate and Potassic Sulphocyanide [he. cit. p. 187, 

 pi. 7. fig. 1). — To one "equivalent" of ferric nitrate successive 

 groups of " equivalents " of potassic sulphocyanide are added, 

 in presence of water; the amount of "red salt produced" is 

 estimated by eye-observations, the liquid being diluted for that 

 purpose up to a standard. The total amount a of red salt thus 

 producible represents in special measure the original unexhausted 

 energy of the nitrate. I have taken each unit of x as represent- 

 ing 25 "equivalents" of potassic sulphocyanide. The equation 



