THE ORIGIN AND PROPAGATION OF DISEASE. 235 



The more essential phenomena of fermentation have beenkuown from 

 time immemorial. If we add to a solution of sugar, or to any clear veg- 

 etable-juice containing sugar, a small portion of yeast, and keep the 

 mixture in a moderately warm place, after a few hours of apparent inac- 

 tivity, certain remarkable changes take place in it : 1. The liquid becomes 

 uniformly turbid. 2. It is more or less agitated by minute bubbles of 

 gas, which are generated in its interior, rise to the surface, and escape 

 there. 3. The sugar gradually disappears from the solution, and alco- 

 hol takes its place. 4. When all the sugar has been thus consumed, the 

 gas-bubbles cease to rise, the liquid again becomes clear and quiescent, 

 its turbid contents being slowly deposited in a whitish layer at the bot- 

 tom; and, 5. This deposit is found to be itself a layer of yeast, often 

 much greater in quantity than that originally added, and capable of 

 exciting the same kind of fermentation in another saccharine liquid. 



Besides this, chemical investigation has shown that the gas evolved 

 is carbonic acid, and that the weight of the sugar which disappears is 

 accounted for, within reasonable limits of accuracy, by that of the car- 

 bonic acid and alcohol produced, with a little glycerine and succinic acid 

 formed at the same time. It is therefore a chemical transformation, in 

 which the elements of the sugar are separated from their combination, 

 and re-arranged to form other non-nitrogenous compounds. 



But it is a chemical change which will not take place spontaneously. 

 It requires the presence of yeast artificially added, or of a natural fer- 

 ment, x)resent in the vegetable-juice. The theory of fermentation for- 

 merly in vogue was, that the nitrogenous matter in solution in the yeast 

 excited, by its own molecular changes, the decomposition of the sugar; 

 taking by itself no direct part in the chemical phenomena, and neither 

 absorbing nor discharging any of the materials of the solution. 



In enumerating these facts I do not always follow the exact chrono- 

 logical order in which they were discovered, nor do I wish to take up 

 your time in alluding to all the details and varieties of fermentation. It 

 will be sufficient for our present purpose to bear in mind simply the main 

 features of the process, namely, the addition of ferment to a saccha- 

 rine liquid, turbidity of the solution, decomposition of the sugar, appear- 

 ance of alcohol and carbonic acid, and, finally, reproduction of the 

 ferment. 



Two hundred years ago Mr. Anthony Leeuwenhoeck was investigat- 

 ing all sorts of natural objects with his newly-constructed microscopes, 

 consisting each " of a very small double convex glass, let into a socket 

 between two silver plates." He examined the blood-globules, the ca- 

 pillary vessels, the spermatic corpuscles, the structure of wood, of hair, 

 of teeth ; and with the same instruments he saw in yeast little glob- 

 ules collected into groups of three or four together.* But he had too 

 many other novelties, all attracting his attention together, to spend 

 much time on any one of them, and he did not learn the nature or spe- 



* " Philosoi)liical Transactions/' 1681, p. 507. 



