84 THE NATURE OF LIFE 



the two substances. The rate of death de- 

 pends on the amount of lime salt added 

 and the death curve can be predicted 

 by means of the mathematical formulae 

 already mentioned. We can also predict 

 the recovery curves obtained when the 

 organism is transferred from any of these 

 solutions to its normal environment. 



In conclusion we may say that in many 

 respects the selective action of the pro- 

 toplasm can be imitated but in others 

 it cannot. The subject is peculiarly dif- 

 ficult because as yet we know very little 

 about the rates at which various sub- 

 stances penetrate. But with improved 

 technique we may hope to get a clear 

 picture of the whole process and to in- 

 terpret it in the light of physics and 

 chemistry. 



VIII 



ELECTRICAL FORCES 



The electrical experiments just de- 

 scribed lead naturally to a consideration 

 of certain other electrical tests. 



