TRANSACTIONS OF SECTION D. 699 



it. When a fermenting liquid comes to contain about 14 per cent, of alcohol, the 

 activity of the yeast ceases, quite independently of whether the sugar is used up or 

 not. In other cases of fermentation the same inhibiting efl'ect of the products of 

 fermentation is met with. Thus lactic fermentation soon comes to an end unless 

 calcium carbonate or some similar substance be added, which removes the lactic 

 acid from the solution as fast as it is formed. 



The other point is that in all fermentations, besides what may be termed the 

 primary products of the process, other bodies are produced. In the case of 

 alcoholic fermentation the primary bodies are alcohol and carbon dioxide ; the 

 secondary, succinic acid and glycerine. Delpino has suggested that these last are 

 residual products derived from that portion of the fermentible matter which is 

 directly applied to the nutrition of the protoplasm. 



Yeast, itself the organism which effects the remarkable changes on which I 

 have dwelt, is somewhat of a problem. It is clear that it is a fungus, the 

 germs of which must be ubiquitous in the atmosphere. It is difficult to believe 

 that the simple facts, which are all we know about it, constitute its entire life 

 history. It is probably a transitory stage of some more complicated organism. 



I can only briefly refer to putrefaction. This is a far more complex process 

 than that which I have traced in the case of alcoholic fermentation. In that 

 nitrogen is absent, while it is an essential ingredient in albuminoids, which are the 

 substances which undergo putrefactive changes. Bat the general principles are 

 the same. Here, too, we owe to Schwann the demonstration of the fact that the 

 effective agents in the process are living organisms. If we put into a flask a 

 putrescible liquid such as broth, boil it for some time, and during the process of 

 boiling plug the mouth with some cotton-wool, we know that the broth will re- 

 main long unchanged, ■\vhile if we remove the wool putrescence soon begins. 

 Tyndall has shown that, if we conduct the experiment on one of the high glaciers 

 of the Alps, the cotton-wool may be dispensed with. We may infer, then, that the 

 germs of the organisms which produce putrefaction are abundant in the lower 

 levels of the atmosphere and are absent from the higher. They are wafted about 

 by currents of air ; but they are not imponderable, and in still air they gradually 

 subside. Dr. Lodge has shown that air is rapidly cleared of suspended dust by an 

 electric discharge, and this, no doubt, affords a simple explanation of the popular 

 belief that thunderous weather is favourable to putrefactive changes. 



Cohn believes that putrefaction is due to an organism called Bacterium Tei'mo, 

 which plays in it the same part that yeast does in fermentation. This is probably 

 too simple a statement ; but the general phenomena are nevertheless similar. 

 There is the same breaking down of complex into simpler molecules ; the same 

 evolution of gas, especially carbon dioxide ; the same rise of temperature. The 

 more or less stable products of the process are infinitely more varied, and it is 

 difficult, if not impossible, to say, in the present state of our knowledge, whether in 

 most cases they are the direct outcome of the putrefactive process, or residual pro- 

 ducts of the protoplasmic activity of the organisms which induce it. Perhaps, on 

 the analogy of the higher plants, in which some of them also occur, we may attri- 

 bute to the latter category certain bodies closely resembling vegetable alkaloids ; 

 these are called ptomains, and are extremely poisonous. Besides such bodies, 

 bacteria undoubtedly generate true ferments and peculiar colouring matters. But 

 there are in most cases of putrefaction a profusion of other substances, which repre- 

 sent the various stages of the breaking up of the complex proteid molecule, and are 

 often themselves the outcome of subsidiary fermentations. " 



These results are of great interest from a scientific point of view. But their 

 importance at the present moment in the study of certain kinds of disease can 

 hardly be exaggerated. I have already mentioned Henle as having first found the 

 true clue to animal histology in the structure of plants. As early as 1840 the same 

 observer indicated the grounds for regarding contagious diseases as due to living 

 organisms. I will state his argument in the words of De Bary, whose ' Lectures 

 on Bacteria,' the last work which we owe to his gifted hand, I can confidently 

 recommend to you as a luminous but critical discussion of a vast mass of difficult and 

 conflicting literature. 



