218 ANIMAL PROTEINS 



combination of the acid with gelatine. On this assumption, 

 mathematical considerations show that the electric charge 

 on the gelatine is given by the expression 2 = \/^ex -f e 2 , where 

 z=the amount of ion taken up, x the concentration of the 

 surrounding solution, and e the excess concentration of 

 diffusible ions in the jelly. 



The property of gelatine and glue which is chiefly used 

 in classifying them into grades of different commercial value, 

 is the strength of the jelly obtained as compared with any 

 arbitrary standard gelatine. An enormous number of other 

 physical tests have been devised, but none are nearly so 

 simple or so reliable. Gelatine is unfortunately very liable 

 to hydrolysis even by water, and long before any amido- 

 acids, etc., have appeared there is a change to a not greatly 

 hydrolyzed product (sometimes called j3 gelatine) which 

 has lost the power of setting to an elastic gel. It is thus the 

 lyophile nature which has been altered, and the fall in 

 elasticity corresponds to the fall in power of compressing 

 water, which is proportional to the concentration of a gelatine. 

 Now the elasticity of a gelatine gel varies as the square of 

 the concentration. Hence if one so arranges the concentra- 

 tions of standard and unknown samples that gels of equal 

 elasticity are obtained, the concentration of a gelatine is the 

 same in both gels, and the relative amounts of a gelatine in 

 the original samples are inversely proportional to the weights 

 used to give gels of equal elasticity. The " strength " of a 

 gelatine or glue is therefore usually stated as the number of 

 grams of a standard gelatine which will yield a gel with 

 elasticity equal to that from 100 grams of the gelatine or 

 glue being tested. Elasticity is matched by lightly pressing 

 with the finger-tips. 



It is also possible to grade samples of gelatine and glue 

 by the estimation of "peptones," whose amount indicates 

 the degree of hydrolysis. Nitrogen is estimated by Kj eldahl's 

 method in the sample and in the precipitate obtained by 

 saturating a solution with zinc sulphate. The difference is 

 calculated as peptones by multiplying by 5*33. Trotman 

 and Hackford say that the results are in the same sequence 



