NITROGEN AND A IK 239 



of organic substances in air has been proved by a similar experiment. 

 If a glass globe be filled with ice and placed in a room where are a 

 number of people, then the presence of organic substances, like albu- 

 minous substances, may be proved in the water which condenses on the 

 surface of the globe. It may be that the miasmas causing infection in 

 marshy localities, hospitals, and in epidemic illnesses proceed from the 

 presence of such substances in the air, as well as from the presence 

 of germs of lower organisms borne in the air as a minute dust. 

 Pasteur proved the presence of such germs in the air by the following 

 experiment : He placed gun-cotton (pyroxylin), which has the appear- 

 ance of ordinary cotton, in a glass tube. Gun-cotton is soluble in a 

 mixture of ether and alcohol, forming the so-called collodion. A cur- 

 rent of air was passed through the tube for a long period of time, and 

 the gun-cotton was then dissolved in a mixture of ether and alcohol. 

 An insoluble residue was thus obtained which actually contained the 

 germs of organisms, as was shown by microscopical observations, and by 

 their capacity to develop into organisms (mould, &c.) under favourable 

 ^conditions. The presence of these germs determines the property of 

 -air of bringing about the processes of rotting and fermentation that 

 is, the fundamental alteration of organic substances, which is accom- 

 panied by an entire change in their properties. The appearance of 

 lower organisms, both vegetable and animal, is frequently to be 

 remarked in these processes. Thus, for instance, in the process of fer- 

 mentation, when, for example, wine is procured from the sweet juice 

 of grapes, a sediment separates out which is known under the name 

 of lees, and contains peculiar yeast organisms. Germs are required 

 before these organisms can appear. 40 They are borne in the air, and fall 

 into such substances from it. Finding themselves under favourable 

 conditions, the germs develop into organisms ; they are nourished at the 

 expense of the organic substance, and during growth change and destroy 

 it, and bring about corruption and rotting. This is why, for instance, 

 the juice of the grape when contained in the skin of the fruit, which 

 allows access of the air but is impenetrable to the germs, does not fer- 

 ment, does not alter so long as the skin remains intact. This is also 

 the reason why animal substances when kept from the access of air 

 may be preserved for a great length of time. Conserves for long sea 



40 The idea of the spontaneous growth of organisms in a suitable medium, although 

 still upheld by many, has since the work of Pasteur and his followers (and to a certain 

 extent of his predecessors) been discarded, because it has been proved how, when, and 

 whence (from the air, water, &c.) the germs appear ; that fermentation as well as infec- 

 tious diseases cannot take place without them ; and mainly because it has been shown that 

 any change accompanied by the development of the organisms introduced may be brought 

 about at will by the introduction of the germs into a suitable medium. 



