AIR AND LIFE. 169 



living organisms require oxygen, and must have it, a large number at 

 all events require to have it offered to them in a combined form. All 

 animals seem to prefer combined oxygen. As to plants, we are in the 

 dark. Certainly free oxygen enters the stomata; but is the oxygen 

 used as such by cells, or does it previously form some compound with 

 some liquid in the plant? We do not know. What we do know, how- 

 ever, is that on our planet and under the present laws of organization 

 and life where oxygen is wanting life is also wanting, and that where 

 oxygen is in excess of the normal ratio life is impaired and after a 

 time destroyed. Such is the main conclusion to be kept in mind. 



We will now consider nitrogen, or azote. The name is significant. It 

 means that this gas is not adequate to maintain life, for we all know 

 that if an animal or plant be placed in an atmosphere containing nitro- 

 gen only, death ensues in a very short time. It should not be inferred 

 that nitrogen is toxic. We inhale a large proportion of it without the 

 slightest inconvenience; but it is inert, and neither burns nor maintains 

 combustion. Its only function in respiration seems to be that of a dilu- 

 ent or moderator. Pure oxygen would be certain death, while, diluted 

 with some amount of nitrogen, it is absorbed only in the requisite pro- 

 portion. Nitrogen here plays the part of water added to wine — a useful 

 part, most certainly, since we could not do without this diluent — but a 

 negative one. But what more could be expected of an inert gas? 



There is, however, a much more important part played by nitrogen 

 in the economy of nature. It is abundant in organisms. It forms a 

 large proportion of our frame and tissues and is most abundant in the 

 atmosphere. Lastly, as shown by Magendie, when animals are deprived 

 of food containing nitrogen, they die. Let us start from this well- 

 established fact, that nitrogenous food is necessary to maintain life in 

 animals — in higher animals at least. This nitrogenous food is, in the 

 long run, provided by plants. While a few plants — lentils, for instance — 

 yield fruits containing a large proportion of nitrogen, the greater num- 

 ber furnish nitrogenous food only by undergoing the transformations 

 which animal digestion effects upon vegetable food — grass, hay, leaves, 

 etc. Some animals require nitrogen in the form of meat, while a greater 

 number are content with that contained in plants; but, upon the whole, 

 nitrogen is always primarily provided by plants. Now, as nitrogen is 

 essential to all animals, how do the plants which provide it manage to 

 incorporate if? Where do they get it? 



The soil contains some amount of nitrates, a proportion of which 

 .it is quite certain that plants absorb, for cultivation always impov- 

 erishes the soil, deprives it more or less of nitrogen, as chemistry shows, 

 and in order to restore its fertility nitrogen must be added to it under 



which are found in the hlood, in the urine, etc., are produced by the cells of 

 the tissues after circulation has entirely ceased, when air and oxygen are no more 

 brought to them. The inference is that animal cells are, according to circum- 

 stances, aerobic or anaerobic. 



