Protoplasm. 43 



When a mixture of gases, such as air, is exposed to the 

 action of a solvent such as water, the proportion of each gas 

 absorbed will depend first, on the coefficient of absorption 

 of the gas, and, secondly, on the proportion of it present 

 in the mixture, the temperature and pressure remaining 

 constant. If the temperature vary, however, the amount 

 of each gas absorbed will vary also, increasing as the tem- 

 perature decreases, and decreasing as it increases. If the 

 pressure vary, the amount of gas absorbed will be directly 

 proportional to the pressure. 



It will be seen that, owing to the very low coefficient of 

 absorption of the three chief gases of the atmosphere, the 

 total amount of the gases dissolved in water is very small 

 indeed. Nevertheless, the oxygen present is sufficient for 

 the maintenance of animal life, as the carbonic acid is 

 sufficient to supply the carbon required by submerged 

 plants. Ammonia, which we have cited as an instance of a 

 gas with a very high coefficient of absorption, is present in 

 such small quantity in the atmosphere that, notwithstanding 

 its extreme solubility, the proportion present in water is 

 quite trifling. Moreover, it is quickly licked out of the air by 

 rain and carried down to the earth, where it at once enters 

 into combination with other substances in the soil, to form 

 ammoniacal salts, or becomes oxidised into nitric acid, this 

 latter substance at once forming nitrates by combination 

 with such bases as potash, lime, and soda. This 'nitrifica- 

 tion ' takes place also in the case of ammonia produced by 

 decaying humus. 



B. Temperature. The extremes of temperature between 

 which life is possible vary greatly according as it is vegetal 

 or animal protoplasm that is under consideration. Gene- 

 rally speaking, life cannot be maintained in either kind of 

 protoplasm under a continued exposure to a temperature 

 of above 50 C., or below o C. In the case of certain 

 extremely simple and minute organisms of doubtful affinities, 

 and of the reproductive cells of certain groups of plants, 



