17 



development on the nature of the sftbstance upon which they 

 grow. The air contains, normally, fungi ; but it contains far more 

 in damp and badly-ventilated stables. It is, therefore, but natural 

 tliat milk should contain them also, since the air cannot be ex- 

 cluded. When milk, on standing, sours or decomposes, we find, 

 therefore, always fungi, whether the process is a normal or an ab- 

 normal one. It is not absolutely, but at least reasonably certain, 

 that these' changes are produced by fermentation, from the fact 

 that Appert's milk-preserving process is successfully applied in 

 dairies. 



The following species of fungi have been found in milk which 

 is undergoing decomposition : 



1. Oidium lactis, now known as mycoderma lactis, milk yeast. 



2. Mucor racemosus Fres. 



3. PeniciUium glaucum Link. 



4. Dictyostelium mucoroides Bref. 



"With these we always find bacteria and various species of 

 vibrio, zooglcea and monades. 



Speefie Gravity. 



Definition : Specific gravity is the figure which indicates the 

 relation of the weight of a body, as compared with the weight of 

 an equal volume of distilled water at the same temperature. The 

 specific gravity of milk, determined at 15° C, depends upon its 

 composition, and varies between certain limits. Albuminates, 

 milk sugar, and salts increase its specific gravity, fat diminishes it. 



The difference in the specific gravity of milk, just after milk- 

 ing, or after standing for some time, or boiling, is, perhaps, due to 

 the air which dissolves in the milk during the milking, when it is 

 thus in thin layers exposed and thoroughly mixed into it. 



The butter globules may be liquid or solid, according to their 

 temperature and composition, and the specific gravity of milk may 

 by this be infiuenced ; but how and to what extent this is done, 

 has not yet been investigated. 



Apparatus for the determination of the specific gravity : 

 The specific gravity of milk is determined — 



1. For scientific purposes, by weighing the milk in a flask of 

 knowil contents (pycnometer). 



