MICROSCOPICAL AND PHYSIOLOGICAL EXAMINATION. 5 



particles of organic and inorganic matter which separate from 

 the solutions often have a deceptive resemblance to various 

 bacterial forms ; and, indeed, it is frequently a matter of the 

 greatest difficulty, if not altogether impossible, for the most 

 experienced observer to determine with certainty whether the 

 small spherical bodies in the field of the microscope are 

 micrococci or particles deposited from the solution. In such 

 doubtful cases it is advisable, before entering on the physio- 

 logical examination described later on, to have recourse to 

 micro-chemical reagents, which often give good preliminary 

 indications. In beer and in nutritive liquids generally which 

 contain albuminoids, these often separate in spherical and 

 thread-like forms ; the starch granules, the dextrines 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 ; the addition of iodine will impart a blue colour 

 to the starch granules which are present, whilst certain 

 dextrines 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 ferric 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 cerevisiae contain a fairly con- 

 siderable quantity of tannic acid during the earlier stages of 

 fermentation. In certain conditions of the yeast-cells' exist- 

 ence a brown-violet coloration of the plasma is produced by 

 iodine, which is interpreted as a glycogen reaction (compare 

 Chapter V.). The so-called nucleus, which is directly visible 

 in many fungi, is in others, and among them the yeast fungi, 

 only observable if the plasma is fixed by means of quickly 

 acting germicides (such as alcohol, picric acid) and then 



