MICROSCOPICAL AND PHYSIOLOGICAL EXAMINATION. 5 



various bacterial forms ; and, indeed, it is often a matter of 

 the greatest difficulty, if not altogether impossible, even for 

 the most experienced observer to determine with certainty 

 whether the small spherical bodies in the field of the micro- 

 scope are micrococci or particles deposited by the solution. 

 In such doubtful cases it is advisable, before entering on the 

 physiological examination described later on, to have recourse 

 to micro-chemical reagents, which often give good pre- 

 liminary indications. In beer and in nutritive liquids generally 

 which contain albuminoids, these often separate in spherical 

 and thread-like forms; the starch granules, the dextrins formed 

 from starch, and even some of the hop constituents may also 

 appear as small spherical bodies. The addition of a small 

 quantity of alcohol, ether, chloroform, acetic acid, soda, potash, 

 etc., is often able to throw some light on the nature of these 

 bodies, the resinous substances being dissolved by the former 

 liquids, whilst the albuminoid matter is acted on more or less 

 by the latter solutions ; the addition of iodine will impart a 

 blue colour to the starch granules which are present, whilst 

 certain dextrins are coloured red by the same reagent. 



In the case of the higher organisms of fermentation yeast 

 and mould-fungi, staining is employed for a different purpose, 

 namely, in order to obtain information concerning the sub- 

 stances which are present in the cell-wall or cell-contents at 

 different stages of their development. On the addition, for 

 instance, of a solution of iron chloride, or any other salt of iron, 

 to cells which contain tannic acid, a bluish-black or green 

 coloration appears in the cells ; in this way it was observed 

 that the cells of Saccharomyces cerevisise contain a fairly con- 

 siderable quantity of tannic acid during the earlier stages of 

 fermentation. If yeast cells are treated with a solution of 

 hsematoxylin or osmic acid, small, sharply-defined, dark- 

 coloured bodies can be seen, which may be regarded as cell- 

 nuclei of the same nature as those generally observed in the 

 cells of the majority of plants without the aid of this treat- 

 ment. 



