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THE AMEBIC AN NATURALIST [Vol. XLII 



at which the reaction is taking place ; and (4) the influence 

 of catalysts upon the reaction. 



For the moment we will confine ourselves to the first 

 two matters, and assume that catalysts are absent and the 

 substances at constant temperature. 



1. The first principle that we have to consider is that 

 which declares that no chemical reaction is really instan- 

 taneous, though the interaction of substances is often so 

 fast that a direct measurement of its rate can not be 

 made; and, further, that every reaction has. its own spe- 

 cific reaction-velocity which distinguishes it from other 

 reactions. This is expressed by giving to each particular 

 reaction a numerical velocity coefficient which is low or 

 high proportionally as the reaction is slow or quick. 



2. This coefficient only expresses the actual experimen- 

 tal velocity when the reacting substances are present in 

 unit concentration, because difference of concentration is 

 just the most important factor controlling the actual re- 

 action-velocity. 



If a solution of a substance A of unit concentration is 

 undergoing change, then to keep this reaction going at its 

 present rate fresh amounts of A must be added continu- 

 ally just to equal the amount removed by the reaction and 

 so keep the substance up to unit concentration. The 

 amount of A that had to be added thus per unit time would 

 give an exact measure of the amount being decomposed, 

 i. e., of the specific velocity of this reaction. 



If the reaction were started with A at double unit con- 

 centration, then twice as much A would have to be added 

 per unit time to keep the reaction velocity constant at the 

 double rate it would have started at. 



And with higher concentrations proportionally more A 

 would have to be added. It is therefore shown that the 

 amount of chemical change going on in unit time is pro- 

 portional to the concentration. This is a most funda- 

 mental principle of chemical mechanics, known as the laiv 

 of mass, and it may be stated thus : the amount of chem- 

 ical change taking place at any time is always propor- 



