To isolate this substance and to prepare it in a state of purity, Pasteur boiled a little yeast with from fifteen to twenty times its weight of water. He then carefully filtered the liquid, dissolved in it about fifty grammes of sugar to the litre, and added to it some chalk. Taking then, by means of a drawn-out tube, from a good ordinary lactic fermentation a trace of the grey matter of which we have just spoken, he placed it as the seed of the ferment in the limpid saccharine solution. By the next day a lively and regular fermentation had set in, the liquid becoming turbid and the chalk disappearing, and one could distinguish a deposit which progressed continually as the chalk dissolved. This deposit was the lactic ferment. Pasteur reproduced this experiment by substituting for the water of the yeast a clear decoction of nitrogenous plastic substances. The ferment invariably presented the same aspect and the same multiplication. These results, however, did not yet satisfy Pasteur. He desired more rigour in a subject of such theoretic importance. Might not the partisans of Liebig's theory argue, if not without subtlety yet with a semblance of justice, that the fermentation was not due to the formation and progressive growth of this feeble nitrogenous globular deposit, but rather to the nitrogenous matter dissolved during the decoction of the yeast used in the composition of the liquor? Up to a certain point it might be maintained that the dissolved matters which had been in contact with the oxygen of the air had been thrown into molecular motion, that this motion had been communicated to the fermentable matter, and that the deposit of the pretended organised ferment was but an accident--one of the physical changes or one of the precipitates so frequently observed in the modifications of albuminoid matters. In the observation of Cagniard-Latour and of Schwann as to the life of the yeast, Liebig saw nothing more. 'One cannot deny,' said he, 'the organisation of the yeast or its multiplication by budding, but these living cells are always associated with other dead cells in process of molecular alteration. It is these molecular motions which communicate themselves to the molecules of the sugar, break them up, and cause them to ferment.' The arguments of Liebig derived great strength from the belief which was shared by all chemists that the cells of yeast perish during fermentation and form lactate of ammonia. On examining this assertion, Pasteur found that not only was there no ammonia formed during alcoholic fermentation, but that even if ammonia were added it disappeared, entering into the formation of new yeast cells. Was not this a proof of the potency of the organised ferment?