246 ROYAL SOCIETY OF CANADA 



which also the rate depends, also change, those of tJie reacting substances 

 growing smaller, and those of the products of the reaction growing 

 greater. 



Thus, as all these circumstances, on which the rate depends, keep 

 changing during the reaction, the rate itself must keep changing 

 throughont the very experiment we are making to study it. This is tlie 

 fundamental diificulty of experiments in chemical kinetics. 



Now as to the way in which this difficulty is overcome ; and first as 

 to the temperature. That is kept constfint during the experiment very 

 feimply by placing tlie beaker in a large tub of water kept at constant 

 temperature, — a thermostat, as it is called. After determining the rate 

 at one temperature, tlie solutions and the thermostat are warmed up to 

 some other temperature, and the experiment is repeated, all the concen- 

 ti-ations and everything but the temperature being tlie same, thus the 

 influence of the temperature is ascertained. In cases where it is not 

 possible to hold the temperature constant during an experiment — as in 

 the study of explosions, for instance — it is much more difficult to inter- 

 pret the results of the experiments, and the study of those reactions is 

 much less advanced than that of those where a thermostat can be em- 

 ployed. 



So much for the temperature; it is kept constant during the experi- 

 ment, and the rates in experiments at different temperatures are com- 

 pared. Now for the concentrations. Can they too be kept constant dur- 

 ing the reaction? No one has yet invented a concentrationstat, so far 

 as I am aware; but as Ilarcourt and Esson pointed out, the concentra- 

 tions will not change much during the experiment, if the quantities of 

 chemicals put into the beaker are large compared to the quantities of tlie 

 same chemicals formed or destroyed during tlie reaction. Now, that is 

 what I did in this beaker; there is very nearly as much oxalic acid and 

 sulphuric acid here yet as (there was before the permanganate was added, 

 beoause the amounts of those two substances put in in the first place 

 were much greater than would react witli the pennanganate used. The 

 concentrations of oxalic acid, and sulphuric acid, therefore have re- 

 mained practically constant during the experiment, and the temperature 

 would have, if tlie bleaker had been stood in a tub. 



This is the way Ilarcourt and Esson worked, and I may express these 

 conditions on the board ^ by writing large letters for B, C, etc., the con- 

 centrations of the reagents of which relatively large quantities were used, 

 and a little "a" for the permangauate. B, C, etc., stay constant, because 

 they are large, and perhaps it will not confuse if I write a large T 



1 See table, p. 248. 



