Protoplasm. 41 



atmosphere, in this manner keeping up that balance of gases 

 in the atmosphere which is necessary for the maintenance 

 of animal life. 



Before the full bearing of these different phenomena 

 can be seen it will be necessary to refer briefly to certain 

 physical facts of the highest importance in the discussion 

 of the relations of animal and vegetal protoplasm firstly, 

 to their inorganic surroundings, and, secondly, to each 

 other. 



SECTION V. CONDITIONS OF THE ENVIRONMENT NECES- 

 SARY FOR THE MAINTENANCE OF LlFE IN ANIMAL AND 



VEGETAL PROTOPLASM. 



A. Composition of the atmosphere. An * empty' room, 

 10 feet everyway, contains 1,000 cubic feet of air, which, if 

 dry and pure, is a mixture of nearly 



790*2 cubic feet of nitrogen. 

 209-4 ,, oxygen. 



0-4 ,, carbonic acid. 



To such air the terms ' normal,' or ' fresh air ' are applied. 

 In addition to these, traces of other gases, such as ammonia 

 (NH : <), and ammoniacal salts, carbonic oxide (CO) and 

 certain other compounds of carbon. The atmosphere, 

 moreover, always contains a certain proportion of water- 

 vapour, which varies as is well known from day to day and 

 from hour to hour. Normally it amounts to from 5 to 

 1 5 per cent. 



If such a room be occupied by an animal, say an adult 

 human individual, it will be found that after about two hours 

 or so (provided there be no addition of fresh air), the atmo- 

 sphere has become unbearable and highly injurious to life, 

 from the presence in it of certain obnoxious gases. The 

 alteration in the composition of the atmosphere is due to 

 the addition to it of a large quantity of carbonic acid gas 

 (and of other gases in less amount) produced in conse- 



