Yeast when macerated in water imparts to it certain soluble nitrogenous materials. The solution so obtained, filtered from yeast globules, is known as _yeast water_.—D. C. R. Footnote 48: It is possible that this greasiness in the cells of the common _mycoderma vini_ arises simply from the composition of the liquid in which it vegetates. It is in saccharine liquids that the submerged _torulæ_ are found; fermented liquids more readily give birth to the forms of _torulæ_ and _mycoderma vini_ which exist as a scum. In all probability, however, there is no radical difference between these two kinds of little cellular plants of aerial growth, the floating _torulæ_ and _mycoderma vini_. CHAPTER IV. The Growth of Different Organisms in a State of Purity: Their _Autonomy_.[49] Our observations in the preceding chapter will have shown that organic liquids, natural or artificial—the wort of beer amongst others—if exposed to contact with the air, rapidly develop various forms of life. This is a natural consequence of the mode of impregnation. The fertility of the liquid depends on the various microscopic germs which are deposited in it by the common air, and these, again, as regards their nature and number, are dependent upon the situation of the vessel containing them, its height above the ground, the time of year, the disturbance of the atmosphere, and other causes. The fortuitous association of other forms, in growths which we believe to be uniform and independent, constitutes one of the principal difficulties that occur in the study of the lower organisms, particularly that of fungoid growths. The fact that the germs of many of these little beings exist in the atmosphere in the form of dust, invisible to the naked eye, or, as such, spread over the surface of the different materials and objects used in experiments, exposes the student to constant risk of wrongly interpreting the results which come under his notice. He has sown a plant, and is observing the course of its development. Without his knowledge, spores of another plant have got mixed with his growth, and germinated. In his ignorance, he will attribute all that he sees, all the changes which he describes and which he sketches, and all the conclusions which he draws, to the one plant which engages his attention. If he is dealing with _bacteria_, _vibrios_, and, generally speaking, the infinite variety of mobile microscopic organisms, his embarrassment will be greater still. Again, inasmuch as the medium which serves as substratum for growths has a considerable influence on the fertility of the germs in contact with it, as well as on their ulterior development, it often happens that germs deposited fortuitously by the particles of dust which fall from the atmosphere or collect on objects are fertile and multiply with rapidity, whilst those which have been directly sown, no matter in what number,