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THE AMERICAN NATURALIST 



[Vol. XLIX 



tion. Since c A , c B and S may be measured at any time, 

 F may be calculated for any action. 38 



The law of molecular concentration or law of mass ac- 

 tion is: In every chemical experiment, the speed of the 

 action at any moment is proportional to the first, or some 

 higher, power of the molecular concentration, at that time, 

 of each interacting substance and to the intrinsic activity 

 (affinity) of the substances. 



2. Van't Hoff's Law 



But the speed of any reaction at any concentration 

 varies with the temperature. In general, the increase in 

 speed is about ten per cent, for each increase of one de- 

 gree Centigrade, or, as it is sometimes expressed, the 

 speed of the reaction is doubled when the temperature is 

 increased ten degrees Centigrade. This is known as Van 't 

 Hoff's law. The actual change in the speed of the re- 

 action may be greater or less than ten per cent, for each 

 change of one degree Centigrade, and is usually expressed 

 by a coefficient. When the coefficient is 1.2 or less, that 

 is, when the change in speed is two per cent, or less for 

 each change of one degree Centigrade, the action is 

 usually considered to be a physical and not a chemical 

 action. When the temperature coefficient is greater than 

 1.2, the action is commonly considered to be a chemical 

 action. No theoretical explanation of Van't Hoff's law 

 of change in speed with change in temperature has so far 

 been advanced. 



These laws apply to reactions which go on at a measur- 

 able speed and which have been called "slow" reactions 

 by the physical chemists. These "slow" reactions are to 

 be distinguished from those reactions which proceed so 

 rapidly that no measurement of their speed at different 

 intervals is possible, or reactions of the explosive type. 



as Smith, "General Inorganic Chemistry/' 1st ed., New York, 1906, p. 251. 



