1 8 PROXIMATE CONSTITUENTS OF PLANTS 



similar manner to that described in 7, p. lo. The 

 ammonia, after liberation with caustic soda, is distilled 

 off as usual, absorbed by standard acid, and estimated 

 by titration. The percentage of nitrogen so found is 



multiplied by 5-8 ( = ), 17-3 per cent, being taken as 



the average percentage of nitrogen in the various pro- 

 teins in wheat, and the result given under the head of 

 protein. 



In the estimation of protein in other substances, different 

 factors are employed, as follows : — 



Oat proteins . . . .6-31 



Maize proteins .... 6-22 

 Flax seed proteins . . • 5*5 



An improved method, whereby a differentiation between 

 the true albuminoid nitrogen and nitrogen existing in other 

 forms is made, is being used to a certain extent in agricultural 

 analysis, especially in America. For practical details of the 

 process, see the section on Feeding- Stuffs, p. 150. 



{U) Amides. 



Amides may be regarded as organic acids in which the 

 hydroxyl of the — COOH group is replaced by the amido group 

 NH2. The amido-radicle is therefore — CONH2, and is almost 

 invariably present, although in different proportions, in proteins 

 and albuminoid bodies, especially the polypeptides ; as, for 

 example, CONH2. NH . CHg. CO . NH . CH2. CONHg, one of the 

 simplest compounds of this class. 



One of the best methods of estimating nitrogen in the amido 

 form depends upon the fact that the — CONHg group, on hydro- 

 lysis with dilute acids, gives —COOH and the ammonium salt 

 of the acid. The ammonia can be readily liberated from this 

 salt by the action of alkalies, and can be estimated by distillation 

 in the usual way. 



Although amido-nitrogen is included under the head of protein 

 nitrogen in ordinary agricultural analysis, the need of distinguish- 

 ing between nitrogen combined in these two forms is evident 

 when their very great difference in feeding value is remembered. 



