292 LOUIS PASTEUR 



iritis* 



though variable, yet, as estimated by weight of product 

 formed, is out of all proportion to the weight of their own 

 substance. These are facts of so great importance, and so 

 intimately connected with the theory of fermentation, that 

 it is indispensable to endeavour to establish them experi- 

 mentally, with all the exactness of which they will admit. 



The question before us is whether yeast is in reality an 

 anaerobian 2 plant, and what quantities of sugar it may 

 cause to ferment, under the various conditions under which 

 we cause it to act. 



The following experiments were undertaken to solve this 

 double problem: We took a double-necked flask, of three 

 litres (five pints) capacity, one of the tubes being curved and 

 forming an escape for the gas ; the other one, on the right 

 hand side (Fie. i), being furnished with a glass tap. We 

 filled this flask with pure yeast water, sweetened with 5 

 per cent, of sugar candy, the flask being so full that there 

 was not the least trace of air remaining above the tap or in 

 the escape tube ; this artificial wort had, however, been it- 

 self aerated. The curved tube was plunged in a porcelain 

 vessel full of mercury, resting on a firm support. In the 

 small cylindrical funnel above the tap, the capacity of which 

 was from 10 cc. to 15 cc. (about half a fluid ounce) we 

 caused to ferment, at a temperature of 20 or 25 C. (about 

 75 F.), five or six cubic centimetres of the saccharine 

 liquid, by means of a trace of yeast, which multiplied rapidly, 

 causing fermentation, and forming a slight deposit of yeast 

 at the bottom of the funnel above the tap. We then opened 

 the tap, and some of the liquid in the funnel entered the 

 flask, carrying with it the small deposit of yeast, which was 

 sufficient to impregnate the saccharine liquid contained in 

 the flask. In this manner it is possible to introduce as 

 small a quantity of yeast as we wish, a quantity the weight 

 of which, we may say, is hardly appreciable. The yeast 

 sown multiplies rapidly and produces fermentation, the 

 carbonic gas from which is expelled into the mercury. In 

 less than twelve days all the sugar had disappeared, and the 

 fermentation had finished. There was a sensible deposit of 

 yeast adhering to the sides of the flask ; collected and dried 



3 Capable of living without free oxygen a term invented by Pasteur. En. 



I 



