THE AMERICAN NATURALIST [Vol. XLII 



cane-sugar then remaining in solution, thus exemplifying 

 the law of mass. 



Such a curve as A in Fig. 1, which changes by a con- 

 stant multiple for successive units of time (here halving 

 itself every n minutes) is known as a logarithmic curve; 

 the velocity of reaction at any moment is exactly indi- 

 cated by the steepness of the curve at that moment; the 

 velocity is greatest at first and it declines to almost zero 

 as the curve approaches the horizontal at the end of the 

 reaction. 



AVhen instead of the decomposition of a single substance 

 we deal with two dissolved substances, A and B, reacting 

 together, then as both of them go on being thus used up, 

 the amount of change must be ever proportional to the 

 mass or amount of A present multiplied by the mass of B 

 present. 



There is a special important case when the amount of, 

 say, B is in very great excess of that amount required to 

 unite with the whole of A. Then all through the slow 

 progress of the reaction the amount of B never becomes 

 reduced enough to make appreciable difference to its 

 mass, and it may be considered as practically constant all 

 along. In such a case the rate of the reaction is found 

 to be proportional simply to the amount of A present, and 

 we get again the curve A, Fig. 1. Here the amount of 

 A may be considered as a limiting factor to the amount of 

 reaction; B being in such great excess never falls low 

 enough to take a practical part in determining the velocity. 



The case of the hydrolysis of cane-sugar in aqueous 

 solution is just such a case. The water itself enters into 

 the reaction, but so little is used up in relation to the 

 enormous excess present that the amount remains prac- 

 tically constant and the rate of hydrolysis of the cane- 

 sugar is determined only by the amount of the cane-sugar 

 present at any moment. 3 



3. We have now shown how the actual amount of chemi- 

 cal change going on in a solution is determined by the 



