i CONDITIONS OF LIFE 21 



duced by heat. Similarly when it is cooled below the 

 ordinary temperature the movements become slower and 

 slower, and at the freezing point (o C.) cease entirely. 

 But freezing, unlike over-heating, does not kill the pro- 

 toplasm, but only renders it temporarily inert ; on thawing, 

 the movements recommence. We may therefore distin- 

 guish an optimum temperature at which the vital actions 

 are carried on with the greatest activity ; maximum and 

 minimum temperatures above and below which respect- 

 ively they cease ; and an ultra-maximum temperature at 

 which death ensues. There is no definite ultra-minimum 

 temperature known in the case of Amoeba. 



The quantity of water present in the protoplasm as water 

 of organization (see p. 5) is another matter of importance. 

 The water in which Amoeba lives, although fresh, always 

 contains a certain percentage of salts in solution, and the 

 protoplasm is affected by any alteration in the density of the 

 surrounding medium ; for instance, by replacing it by dis- 

 tilled water and so reducing the density, or by adding 

 salt and so increasing it. The addition of common salt, 

 (sodium chloride) to the amount of 2 per cent, causes 

 Amoeba to withdraw its pseudopods and undergo a certain 

 amount of shrinkage : it is then said to pass into a con- 

 dition of dry-rigor. Under these circumstances it may 

 be restored to its normal condition by adding a sufficient 

 proportion of water to bring back the fluid to its original 

 density. 



In this connection it is interesting to notice that the dele- 

 terious effects of an excess of salt are produced only when 

 the salt is added suddenly. By the very gradual addition of 

 sodium chloride Amoebae have been brought to live in a 4 

 per cent, solution, i.e., one twice as strong as would, if added 

 suddenly, produce dry-rigor. 



