CONDITIONS OF LIFE 21 



duced by heat. Similarly when 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 protoplasm, but only 

 renders it temporarily inert ; on thawing the movements re- 

 commence. We may, therefore, distinguish an optimum 

 temperature at which the vital actions are carried on with 

 the greatest activity ; maximum and minimum tempera- 

 tures above and below which respectively 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 only produced 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. 



