of Chemical Cliange. 445 



the rate at which ferrous-sulphate solution is oxidized by 

 potassic chlorate is retarded in a remarkable manner by the 

 addition of various sulphates. The principal results obtained 

 were these — that the amount of such retardation is propor- 

 tional to the quantity of the sulphate added, and that certain 

 groups of analogous sulphates produce, for equal weights, the 

 same retardation-effect. For instance, equal weights of the 

 sulphates of sodium, potassium, and ammonium were found 

 to produce the same amount of retardation ; so also did the 

 potash- and ammonia-alums ; whereas the sulphates of zinc 

 and magnesium, although classed together as analogous salts 

 in the same sense as those of the alkali metals, produced dif- 

 ferent retardations. The natural inference that was drawn 

 from these experiments was, that the study of retardation 

 might afford a means of classifying chemical substances on a 

 dynamical basis, and of determining for each salt or group of 

 salts a numeric, the coefficient of retardation, of a character 

 somewhat similar to what Mills* has termed the "bergmannic" 

 of a salt. Owing to the small number of soluble sulphates 

 that can be employed in the above reaction for studying their 

 retardation-effects, it is very limited in its application ; conse- 

 quently search was made for other reactions that could be 

 employed for the same purpose, of such a nature that nitrates 

 or chlorides could be experimented with. Of the several 

 reactions that were tested for this purpose, it was found that 

 the oxidation of ferrous chloride by potassic chlorate was by 

 far the best, being analogous in all respects to that of the 

 oxidation of ferrous sulphate, and capable of being rendered 

 as quick or as slow, by alteration of conditions, as might be 

 necessary for the purpose in view ; besides, the list of soluble 

 chlorides whose retardation-effects could be studied is a com- 

 paratively large one. The results, however, that have been 

 obtained with these salts are such as have modified somewhat 

 the author's original notions concerning retardation and the 

 molecular movements that are generally supposed to take 

 place in a system undergoing chemical change. 



It has been usual, in establishing the formulse that are em- 

 ployed in the study of the rate of chemical change, to ignore 

 the fact that the products of the reaction, however inactive 

 they may be in the chemical sense, may retard the rate of the 

 change considerably. In the experiments with ferrous sul- 

 phate referred to, it was shown that the introduction of so 

 small a quantity as one gram of the sulphates of the alkali 

 metals in 260 cubic centims., the volume of the experimental 



* Phil. Mag. [o] i. 



