56 PASTEUR: THE HISTORY OF A MIND 



II 

 FROM LAVOISIER TO GAY-LUSSAC 



Here we are able to point out, as we have done with 

 respect to the introduction of polarization into chemistry, 

 the fruitful power of a new instrument entering into a 

 science which had previously not known it or had ne- 

 glected it. It is to the introduction of the balance 

 into chemistry that Lavoisier owes his glory ; it had served 

 him well in the solution of other problems: it solved 

 also the problem of fermentation. Lavoisier placed on 

 the pan of a balance a vessel filled with water to which 

 he had added a given weight of sugar and a little yeast of 

 beer. From the loss of weight undergone by this vessel 

 at the end of the fermentation, he inferred the weight 

 of the carbonic acid 1 liberated during the process of the 

 phenomenon. He then separated the alcohol by dis- 

 tillation, weighed it, and found that the sum of the 

 weights of the alcohol and the carbonic acid gave very 

 nearly the original weight of the sugar. The conclusion 

 is easy to draw: the sugar simply breaks up into alcohol 

 and carbonic acid; there are no other normal products 

 of the transformation. 



But there is more than that in the experiment of 

 Lavoisier. The relation which exists between the weight 

 of the sugar on the one hand and that of the alcohol 

 and carbonic acid on the other, ought also to be verified 

 individually for each one of the elements of these bodies. 

 The carbon of the sugar ought, for example, to be found 

 entire in that of the alcohol and the carbonic acid; the 

 same should be true for the hydrogen and the oxygen. 



1 Following the usage of Pasteur and his opponents and all the older 

 writers, this book, which is an interpretation, calls the dissolved gas and 

 the free gas, indifferently, carbonic acid. Trs. 



