240 PKINCIPLES OF CHEMISTRY 



voyages are preserved in this way. 41 Hence it is evident that however 

 infinitesimal the quantity the germs carried in the atmosphere may be, 

 still they have an immense significance in nature. 4 ' 2 



Thus we see that air contains a great variety of substances. The 

 nitrogen, which is found in it in the largest quantity, has the least 

 influence on those processes which are accomplished by the action of air. 

 The oxygen, which is met with in a lesser quantity than the nitrogen* 

 on the contrary, takes a very important part in a number of reac- 

 tions ; it supports combustion and respiration, it brings about corruption 

 and every process of slow oxidation. The part played by the moisture 

 of air is known to everyone. The carbonic anhydride, which is met 

 with in still smaller quantities, has an immense significance in nature, 

 inasmuch as it serves for the nourishment of plants. The importance 

 of the ammonia and nitric acid is immense, because they are the sources 

 of the nitrogenous substances comprising an indispensable element 

 in all living organisms. And, lastly, the infinitesimal quantity of germs 

 evinces their significance in a number of processes. Thus it is not the 

 quantitative but the qualitative relations of the component parts of the 

 atmosphere which determine its importance in nature. 43 



Air, being a mixture of various substances, may suffer considerable 

 changes in consequence of incidental circumstances. It is particularly 

 important to remark that change in the composition of air which takes 

 place in dwellings and in various localities where human beings have to 

 remain during a lengthy period of time. The respiration of human 

 beings and animals alters the air. 44 A similar deterioration of air is 

 produced by the influence of decomposing organic substances, and 



41 In further confirmation of the fact that putrefaction and fermentation depend on 

 germs carried in the air, we may cite the circumstance that poisonous substances de- 

 stroying the life of organisms stop or hinder the appearance of the above processes. 

 Air which has been heated to redness or passed through sulphuric acid no longer contains 

 the germs of organisms, and loses the faculty of producing fermentation and putrefaction. 



42 Their presence in the air is naturally due to the diffusion of germs into the atmo- 

 sphere, and owing to their microscopical dimensions they, as it were, hang in the air in 

 virtue of their large surfaces compared to their weight. In Paris the amount of dust 

 hanging in the air equals from f> (after rain) to 23 grams per 1000 c.m. of air. 



45 w e see similar cases everywhere. For example, the predominating mass of sand 

 and clay in the soil takes hardly any chemical part in the economy of the soil in respect 

 to the nourishment of plants. The plants by their roots search for substances which are 

 diffused in comparatively small quantities in the soil. If a large quantity of these 

 nourishing substances be taken, then the plants will not develop in the soil, just as animals 

 die in oxygen. 



44 A man inbreathing burns about 10 grams of carbon per hour that is, he produces 

 about 880 grams, or (as 1 c.m. of carbonic anhydride weighs about 2000 grains) almut 

 ^ c.m. of carbonic anhydride. The air coming from the lungs contains 4 p.c. of carbonic 

 anhydride by volume. The exhaled air acts as a direct poison, owing to this gas and to 

 other impurities. 



