of Chemical Change. 455 



of oxidation. In the case of the sulphates, the retardation- 

 effects of the three latter stand in order of magnitude thus : 

 MgS0 4 > ZnS0 4 > CdS0 4 ; and the accelerations of the chlorides 

 are seen to be in the same order — MgCl 2 > ZnCl 2 > CdCl 2 . 



In attempting to form a mental image of the state of mat- 

 ters in a chemical system undergoing change, it is customary 

 to think of the moving parts or molecules, their velocities of 

 translation, and the influence of various agencies, such as heat 

 &c, increasing the velocities of the molecules and multiplying 

 the chances of collision in a given time. Employing such 

 language, it is easy to account for the retardation occasioned 

 by ihe presence of chemically inactive substances in the system 

 by saying that, by the motions of the molecules of such bodies, 

 they interfere with the movements of the chemically active 

 molecules by coming into collision with them ; in fact, con- 

 tinually getting in the way of the molecules of the several 

 bodies undergoing chemical change, and so diminishing the 

 number of impacts in a given time between the latter. 



To such an explanation, however^ the experiments in the 

 first part of this paper are open to a grave objection. It 

 might be argued, that the introduction of a sulphate into a 

 solution of ferrous chloride in presence of much free HOI 

 would give rise to such double decompositions as would 

 account for the retardation observed in the rate of oxidation. 

 If this were so, however, it is difficult to see how the retarda- 

 tion is the same in amount for equal weights of different 

 sulphates, as well as proportional to the amount of the salt 

 present, being the same result as was obtained with sulphates 

 in a ferrous sulphate solution. In the experiments with 

 chlorides no such double decompositions could occur, but here 

 the results are anomalous. 5 grams of sodic chloride produce 

 no effect whatever, whilst the same weight of the sulphate in 

 ferrous chloride would give a retardation of 85 per cent., and in 

 ferrous sulphate a retardation of 50 per cent. If retardation- 

 effects are to be interpreted on the hypothesis of intermolecular 

 movements and interference, and consequent diminution in 

 the number of impacts, how comes it that so much sodic chlo- 

 ride produces no effect, whereas the same weight of potassic 

 chloride causes a retardation of 22 per cent.? And, considering 

 the subject in the same light, more remarkable still are the 

 effects of magnesic, zincic, and cadmic chlorides, which pro- 

 duce an acceleration of the rate of oxidation, or increasing the 

 number of collisions of the active bodies in a given time by 

 an amount varying from 6 to 20 per cent. It seems difficult 

 to account for such results on the hypothesis of interdiffusion 

 and simple contact between the active molecules. Instances 



2K2 



