144 STUDIES ON FERMENTATION. 



substances. The only difference that may occur in these pheno- 

 mena is a certain amount of retardation, in cases where the 

 yeast does not reach the saccharine matters in a perfectly 

 natural form, inasmuch as it then requires a certain time to get 

 itself together before it can begin to act. 



It is necessary, indeed, that sugar should be present ; for if 

 we abstracted by some means or other from the must or dough 

 all the sugar contained in it, without touching the other 

 constituents, the addition of yeast would produce no gas. Every- 

 thing would remain quiet until the moment when signs of a 

 more or less advanced putrefaction showed themselves. Yeast 

 is one of the most putrescible of substances, and it is worthy of 

 notice that its alteration is also the consequence of the formation 

 of one or more ferments, very different, however, from that of 

 which we are speaking. As for the nature of yeast, the micro- 

 scope has taught us what it is. That marvellous instrument, 

 although still in its infancy, enabled Leuwenhoeck, towards the 

 close of the 17th century, to discover that yeast is composed of 

 a mass of cells. In 1835 Cagnard-Latour and Schwann took 

 up Leuwenhoeck's observations, and by employing a more per- 

 fect microscope, discovered that these same cells vegetate and 

 multiply by a process of gemmation. Since then the physical 

 and chemical phenomena already mentioned, such as the raising 

 of the mass, the liberation of carbonic acid gas, and the forma- 

 tion of alcohol, have been announced as acts probably connected 

 with the living processes of a little cellular plant, and subsequent 

 researches have confirmed these views. 



In introducing a quantity of yeast into a saccharine wort, it 

 must be borne in mind that we are sowing a multitude of 

 minute living cells, representing so many centres of life, capable 

 of vegetating with extraordinary rapidity in a medium adapted 

 to their nutrition. This phenomenon can occur at an}' tempera- 

 ture betweeen zero and 55° C. (131° F.), although a temperature 

 between 15° C. and 30° C. (59° F. and 86° F.) is the most 

 favourable to its occurrence. 



As regards the rapidity of the budding, the following observa- 



