No. 502] CHEMICAL MECHANICS IN LI VING PLANT 637 



plants, the permanent growth movements of tropistic cur- 

 vatures, or the complex changes of plant-form and de- 

 velopment that result from present and past variations of 

 external conditions. 



The strength and the weakness of the conception lie in 

 its extraordinary lack of particularity. When an irritable 

 cell responds to a stimulus by a reaction nothing is im- 

 plied about the mechanism connecting the cause and the 

 effect, and nothing even about the relative magnitudes 

 of these, but all this is left for special research on 

 the case under consideration. The one natural chain of 

 cause and effect that is recognized to be outside this com- 

 prehensive category is that rather uncommon one in which 

 a definite amount of energy of one kind is turned into an 

 equivalent definite amount of energy of another. Here 

 we have a direct ' ' equation of energy, ' ' whereas in a reac- 

 tion to a stimulus we are said to have typically an "un- 

 loosing" effect— a liberation of potential energy by a 

 small incidence of outside energy, as in the classical an- 

 alogies, drawn from completely comprehended non-living 

 things, of a cartridge exploded by a blow, or the liberation 

 into action of a head of water by the turning of a tap. 



So elastic a conception may be easily stretched to fit al- 

 most any sequence of phenomena with the apparent close- 

 ness that argues a bespoken garment. AVe must therefore 

 be critically on our guard against cases of such sartorial 

 illusion. 



The Principles of Chemical Mechanics. 



That my consideration of particular cases may be intel- 

 ligible it seems necessary that I devote a few minutes 

 to outlining the four quantitative mechanical principles 

 which govern every single chemical reaction, though much 

 that I have to say has been drawn from elementarv books 

 on physical chemistry. 



These four principles are concerned with (1) the nature 

 of the reaction in question; (2) the amount of reacting 

 substances that happen to be present ; (3) the temperature 



