NITROGENOUS COMPOUNDS 67 



per cent, of nitrogen. The percentage of sulphur varies 

 within wider limits ; keratin contains nearly 5 per cent, of that 

 element, haemoglobin only about 0*4 per cent. 



In .analyses of cattle foods, the " albuminoids " are usually 

 estimated by multiplying the nitrogen by 6*25. This factor 

 is based on the assumption that the compounds contain, on 

 the average, 16 per cent, of nitrogen. [100 -f- 16 = 6*25.] 



Molecular Weight. The simplest formula for serum 

 albumin, calculated from the percentage composition given 

 above, is C75H 12 oN 19 O 2 3S. This corresponds to a molecular 

 weight of 1686. It is tolerably certain, however, that the 

 molecule contains more than one atom of sulphur ; judging from 

 the dissociation products, there are at least four, and some 

 authorities hold that there are probably six, atoms of sulphur 

 in the molecule. In the latter case, the formula would be 

 Q5oH72o N ii4Oi38S6> an( i tne molecular weight 10,116. 



The molecular weight estimated by Raoult's method the 

 lowering of the freezing point of solvents is 14,270; but this 

 result is not considered reliable, and it cannot be checked 

 from the boiling point of the solvents as the proteins coagulate. 

 There is no room for doubt, however, that the molecular weight 

 of all proteins even of peptones which are among the simplest 

 must be very high. 



Salts. Of the proteins that are soluble in water, the great 

 majority are neutral to litmus. Some, however, are alkaline 

 and others are acidic. The latter displace carbon dioxide 

 from carbonates. But whatever reaction they exhibit towards 

 litmus, they all unite both with acids and with bases to form 

 salt-like compounds. 



It will be shown presently that the proteins are believed to 

 be aggregates of amino-acids. It is probable that the acidic 

 and basic radicles are present in equivalent proportions in the 

 neutral proteins, but not in those which have an acid or 

 alkaline reaction. The amphoteric character is common to 

 all amino-acid bodies. Glycocine, for example, forms salts 

 both with HC1 and with NaOH (p, 50). 



The sulphates, nitrates, and chlorides of the proteins are 

 soluble to some extent in alcohol, and they are all more readily 



