I CONDITIONS OF LIFE 21 



show an increased activity, then become more and more 

 sluggish, and at about 30° — 35° C. cease altogether, re- 

 commencing, however, when the temperature is lowered. 

 If the heating is continued up to about 40° C. the animal- 

 cule is killed by the coagulation of its protoplasm (see p. 5) : 

 it is then said to suffer heat rigor or death-stiffening pro- 

 duced by heat. Similarly when it is cooled below the 

 ordinary temperature the movements become slower and 

 slower, and at the freezing point (0° 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 distinguish 

 an optimum temperature at which the vital actions are carried 

 on with the greatest activity ; maximum and minimum tem- 

 peratures 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 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 distilled 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 two per cent, causes Amoeba to withdraw its 

 pseudopods and undergo a certain amount of shrinkage : it 

 is then said to pass into a condition of dry-rigor. Under 

 these circumstances it may be restored to its normal con- 

 dition 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- 



