88 ALBUMINOUS MATTERS. 



reduction and deposit of the copper oxide. The peculiar action above 

 described depends upon the presence of albuminose, and not upon that 

 of any original ingredient of the gastric juice ; since it is not exhibited 

 by the perfectly clear and colorless juice, obtained from the empty 

 stomach of the fasting animal by irritation of the mucous membrane 

 with a glass rod or metallic catheter ; while the same fluid, if macerated 

 for a time with finely chopped meat at a temperature of 38 (lOQo F.), 

 will be found to have acquired the property in a marked degree. Gas- 

 tric juice, furthermore, drawn from the stomach after digestion has 

 been going on for half an hour or more, always contains a certain 

 quantity of albuminose, and consequently interferes, as above described, 

 with Trommer's test. 



Albuminose, if present in notable quantity, will also interfere with the 

 mutual reaction of starch and iodine. If gastric juice, containing albu- 

 minose, be mingled with an equal volume of iodine water, and a solution 

 of starch be subsequently added, no blue color is produced ; though if 

 the iodine water be added in excess, or if the tincture of iodine be used 

 instead of its aqueous solution, the superabundant iodine then com- 

 bines with the starch, and produces the ordinary blue color. This 

 property, like that described above, is not possessed by pure, colorless 

 gastric juice, taken from the empty stomach, but is acquired by it on 

 being digested with albuminoid substances. 



Accordingly, in testing for the presence of glucose in fluids which are 

 liable to contain albuminose or other organic substances of similar 

 character, the precaution must always be adopted of first eliminating 

 the albuminous matters which might interfere with the test. This may 

 be done in either of two ways : first, by evaporating the fluid to dryness 

 over the water bath, and extracting the dry residue with alcohol, which 

 takes up the sugar, but leaves behind the albuminous matters. The 

 alcoholic solution may then be filtered and evaporated, and the evapo- 

 rated residue dissolved in water, when it will respond to Trommer's test 

 if glucose be present. Or, secondly, the fluid may be treated with ani- 

 mal charcoal, which retains the albuminous matters, and allows the 

 glucose to pass through in watery solution. 



Caseine, 



This is the principal albuminous ingredient of milk, the only animal 

 fluid in which it is certainly known to exist. It is called caseine, be- 

 cause, when solidified, it forms the substance of cheese. It is not 

 affected by a boiling temperature, but coagulates on the addition of any 

 of the dilute acids, organic as well as mineral, and of magnesium sul- 

 phate. These characters are sufficient to distinguish it from albumen. 

 It is also coagulated by a temperature of 30 (860F.),'by contact with 

 gastric juice, or an infusion of rennet, the mucous membrane of the 

 fourth stomach of the calf. In solution in neutral fluids it rotates the 

 plane of polarization to the left 80. Caseine is an important article 

 of food, being the principal nutritious ingredient in preparations of milk. 



