Page:Popular Science Monthly Volume 20.djvu/529

Rh we are to proceed in order to arrive at the object to be accomplished.

For a series of the most important disclosures, we are indebted to those investigations which have been devoted to the life-processes of the lowest organisms, consisting of single cells. Not that we have here progressed any further than elsewhere, but we have attained the certainty that the most important processes take place in the cells themselves, and that substances are formed which are able to produce powerful chemical changes. The processes to be first considered are those included under the general name of fermentation.

The first known process of fermentation is that which cane-sugar undergoes. A fungus, the so-called yeast, converts the sugar into glucose, and then decomposes this into alcohol and carbonic acid, with the simultaneous formation of small quantities of other products, such as succinic acid, glycerine, etc. The amount of yeast is not increased unless other substances, particularly nitrogenous organic and certain inorganic salts required in building up the body of the cells, are present. From this it appears that various chemical processes take place within the cell, the most striking of which is the decomposition of the sugar. How this takes place is a question which has been largely discussed. From the circumstance that carbonic acid and alcohol are produced in such proportions that the quantities of carbon, hydrogen, and oxygen contained in them are just sufficient to form the sugar, one might suppose that the decomposition is very simple; but, notwithstanding all attempts, the same reaction has never been produced without the aid of yeast. Two hypotheses have been proposed to explain this phenomenon: the one assumes that the yeast-cell contains a ferment which splits the glucose directly into carbonic acid and alcohol, the same as invertin changes cane-sugar into dextrose and lævulose; the other, on the contrary, considers the decomposition of the grape-sugar as an effect of the vital action of the yeast-cells, comparable to the conversion of albumen into carbonic acid, water, and urea in the organism of mammals.

In order to form an opinion of the value of these hypotheses, we will briefly indicate the points of view whence they have been projected. In the first theory consideration is given chiefly to the similarity which the outward appearance of the yeast fermentation has with certain chemical processes, such as the decomposition of sugar by invertin. In both cases the presence of a small amount of yeast or invertin suffices to decompose a comparatively large quantity of sugar, without any apparent change being produced in the yeast or in the invertin. This has given rise to the supposition of the existence of an alcohol ferment in the yeast; a theory which will be verified, when the suspected ferment shall be separated from the yeast, as has already been done with the invertin. All experiments, however, which have been made in this direction have resulted negatively, and hence the