C. A. WATKINS ON YEAST AND OTHER FERMENTS. 163 



plies, for in such a solution the yeast positively lives on its own 

 substance : this has been proved by Pasteur, in the following 

 manner : — " He took a quantity of washed yeast, and divided it 

 into two equal portions, — one of these was placed in a solution of 

 pure sugar, the other portion was boiled in water, the decoction 

 filtered, and the filtrate added to a similar solution of sugar, to 

 which a very minute quantity of fresh yeast was added. In the 

 first case 12 parts of sugar were converted into alcohol in six days, 

 when the yeast became exhausted. In the second case the liquid 

 became turbid ; fresh yeast was formed at the expense of the 

 azotised matter derived from the boiled yeast, and ten parts of 

 sugar were fermented in nine days." 



Some years ago, when experimenting on bread making, I was 

 much puzzled by finding that when the yeast was thoroughly 

 washed the sponge did not rise so quickly, nor was the bread so 

 light as when made with yeast as received from the brewery. I have 

 since learned that a portion of the yeast is soluble in water, and that 

 when it has been dissolved out by washing, the yeast is less active ; 

 on exposure to the atmosphere, however, it recovers its activity. 



Yeast causes a curious and important change to take place when 

 added to a solution of cane sugar, converting it into fruit sugar by 

 causing it to combine with one equivalent of water, during which 

 operation the solution increases in specific gravity. This trans- 

 formation is attributed to the soluble portion of the yeast ; but be 

 this as it may, some of it is evidently destroyed by the process, as 

 a larger proportion of yeast is required to convert cane sugar into 

 alcohol than grape sugar. It is a fact scarcely known to brewers, 

 who use it, that cane sugar cannot be fermented into alcohol ; for 

 although when yeast is added to a cane sugar solution the vinous 

 fermentation eventually ensues, it nevertheless does not commence 

 until the yeast, without any apparent change in itself, has transformed 

 the whole of the cane sugar into fruit sugar. The progress of this 

 transformation may be witnessed by polarized light : the cane 

 sugar producing a right hand rotation of the ray=73°, while the 

 fruit sugar causes a left hand rotation of 26°. 



I have one more observation to make in reference to yeast. 

 When it has been kept some days, of course, according to tempera- 

 ture, it loses the pleasant smell it had when fresh, and acquires 

 some fermentive properties, which, as far as I am aware, have not 

 received much attention. It is well known to brewers that if the 

 yeast be allowed to stand on the beer for a day or two after fer- 



