1897-98.] Dr W alker on the Velocity of Graded Actions. 23 
remain constant. In actual chemical processes, however, it is 
only rarely that we find strict accordance with the theory. Most 
frequently we can account for the divergences by the disturbing 
effect of secondary reactions, change in the medium as the reaction 
progresses, etc., and can occasionally exclude these disturbing 
factors by suitably selecting the experimental conditions. A great 
many reactions, nevertheless, give numbers which cannot be 
brought into harmony with the theory given above, unless they 
are considered from a standpoint essentially different from that 
adopted in forming the ordinary chemical equations devised to 
express them. Thus, for example, the oxidation of ferrous sul- 
phate by potassium chlorate in presence of excess of sulphuric acid 
is expressed by the equation, 
KC10 3 + 6FeS0 4 + 3H 2 S0 4 = KC1 + 3Fe 2 (S0 4 ) 3 + 3H 2 0 . 
This equation would show that the concurrence of seven molecules, 
if we exclude those of the sulphuric acid, is necessary for the 
reaction to take place. But Hood * found that the reaction obeys 
the bimolecular formula under the most varied conditions. The 
action of bromic acid on hydriodic acid leads to a similar dis- 
crepance. The chemical equation is 
HBr0 3 + 6HI = HBr + 3H 2 0 + 3I 2 , 
which assumes that seven molecules interact. The actual rate at 
which the action proceeds by no means conforms to this conception 
of it. If we adopt the theory of electrolytic dissociation, the 
chemical equation becomes still more complex, and the divergence 
between the theoretical and the actual course of the reaction still 
more marked. Van’t Hoff himself studied two specially simple 
cases of this kind, viz., the decomposition of arseniuretted hydrogen 
and of phosphuretted hydrogen. We usually express these decom- 
positions by means of the following equations — 
4AsH 3 = As 4 + 6H 2 , 
4PH 3 = P 4 + 6H 2 , 
indicating that the reactions are quadrimolecular. Van’t Hoff’s 
experiments showed, however, that both obey very exactly the 
* Phil. Mag., 5th series, vi. 371 ; viii. 121. Cf. also Noyes and Wason, 
Zeitschrift fur physikalische Chemie, xxii. 210. 
