NITROGENOUS COMPOUNDS 81 



conclusion is the more important because haemoglobin is 

 easily crystallised, and so can be obtained in a pure state for 

 experiment ; but it affords no information regarding the mole- 

 cular weight of globin as it is not known whether one or 

 several molecules of the latter are united with haematin, the 

 chromogenic radicle. 



The gluco-proteins contain no phosphorus, and they are 

 much more stable than the nucleo-proteins. The prosthetic 

 (carbohydrate) group can be dissociated from the protein only 

 by prolonged boiling with mineral acids or alkalis. As a 

 consequence of this drastic treatment, the proteins are resolved 

 to proteoses or even to amino-acids and the carbohydrate 

 is obtained as glucosamine, C 6 H n O 5 NH 2 . It is impossible, 

 therefore, to tell in what form these two products were 

 originally present in the gluco-protein molecule; but it is 

 highly probable that both result from the disintegration of 

 radicles of much greater complexity. 



The gluco-proteins are represented chiefly by the mucins 

 and mucoids, which occur in the slimy fluids found on the 

 surfaces of all mucous membranes, e.g. those of the intestines. 

 These fluids serve as lubricants, and it is the gluco-proteins 

 which impart the slimy character to them. 



Quantitative Determination of Protein. Kjeldahl's 

 method is now generally employed for the quantitative 

 determination of nitrogen in organic matter, e.g. feeding stuffs. 

 In this process, the substance is boiled with concentrated 

 sulphuric acid until the carbon is completely oxidised and the 

 nitrogen is converted into ammonia which remains in com- 

 bination with the acid. Excess of alkali is then added to 

 liberate the ammonia which is distilled off, and the amount of 

 standard acid which it neutralises is determined by titration 

 in the usual way. The nitorgen multiplied by the factor 6*25 

 (p. 67) gives the total nitrogenous matter or crude protein. 



In practically all feeding stuffs, except those composed of 

 seeds, e.g. oil cake, it is necessary to discriminate between the 

 true protein and the so-called amides (amino-acids, etc.). To 

 do this, the substance is treated with a hot 4 per cent, solu- 

 tion of phenol aqueous or alcoholic to which a small amount 



G 



