invalidate the conclusion that all the nitrogen of the cells of the alcoholic and lactic ferments was derived from the nitrogen in the ammoniacal salts, and that all the carbon of those ferments was taken from the sugar, since, in the medium employed in our experiment, the sugar was the only substance that contained carbon. Liebig carefully abstained from noticing this fact, which would have been fatal to the very groundwork of his criticisms, and thought that he was keeping up the appearance of a grave contradiction by arguing that we had never obtained a simple alcoholic fermentation. It would be unprofitable to dwell longer upon the subject of the difficulties which the propagation of yeast in a saccharine mineral medium formerly presented. As a matter of fact, the progress of our studies has imparted to the question an aspect very different from that which it formerly wore; it was this circumstance which emboldened us to offer, in our reply to Liebig before the Academy of Sciences in 1871, to prepare, in a saccharine mineral medium, in the presence of a commission to be appointed by our opponent, any quantity of ferment that he might require, and to effect the fermentation of any weight of sugar whatsoever. Our knowledge of the facts detailed in the preceding chapters concerning pure ferments and their manipulation in the presence of pure air, enables us to completely disregard those causes of embarrassment that result from the fortuitous occurrence of the germs of organisms, different in character from the ferments, introduced by the air or from the sides of vessels, or even by the ferment itself. Let us once more take one of our double-necked flasks (Fig. 22, p. 110), which we will suppose is capable of containing three or four litres (six to eight pints). Let us put into it the following:— Pure distilled water. Sugar candy 200 grammes. Bitartrate of 1·0 “ potassium ” “ ammonia 0·5 ” Sulphate of ammonia 1·5 “ Ash of yeast 1·5 ” [1 gramme = 15·43 grains.] Let us boil the mixture, to destroy all germs of organisms that may exist in the air or liquid or on the sides of the flask, and then permit it to cool, after having placed, by way of extra precaution, a small quantity of asbestos in the end of the fine, curved tube. Let us next introduce a trace of ferment into the liquid, through the other neck, which, as we described, is terminated by a small piece of india-rubber tube closed with a glass stopper. Here are the details of such an experiment:—