8 April, 1907] Elements of Animal Physiology. 233 



Proteins can be precipitated by — 



{a) Salts of heavy metals, for instance, mercuric chloride (corrosive 

 sublimate), copper sulphate (blue stone), lead acetate (sugar of lead), iron 

 chloride, &c. 



{h) Strong mineral acids, notably nitric acid. 



{c) Addition of ammonium sulphate to saturation point j the protein is 

 here thrown out of solution in the same manner that soap is curdled bv 

 adding common salt to its solution. 



{d) Certain reagents, like tannin or i)icric acid; the precipitation here 

 produced depends upon the presence of the hexone amino-acids in the nro- 

 tein complex. 



(<-) Alcohol, which precipitates all proteins except a few occurring in 

 the plant kingdom. 



(/) To the above we might add the coagulation produced by boiling ; 

 the number of proteins thus precipitated is small, but the two chief animal 

 proteins (albumens and globulins) possess this character. 



In addition to these precipitation re-actions, proteins vield a number 

 of colour tests when treated with certain re-agents — 



(rt) Due to the presence of tyrosin. — When protein is warmed with 

 nitric acid, a yellow colour is produced ; if the mixture is then cooled and 

 ammonia added, an orange tint is, developed. This reaction is frequentlv 

 observed with young chemists who spill nitric acid over their fingers, a 

 yellow stain is formed, which darkens with ammonia. 



{h) Due to the presence of tryptophane. — If to a solution of protein 

 a small quantity of strong acetic acid (which has been kept some time) is 

 added, and then large excess of strong sulphuric acid, a splendid purple 

 colour is produced. 



(c) Due to the presence of cystin. — If protein, strong potash, and lead 

 acetate be boiled together, the mixture darkens owing to the formation of 

 sulphide of lead. The darkening of a silver egg-spoon is similarlv due to 

 the formation of silver sulphide, the sulphur being derived from the cvstin 

 of the albumen. 



{d) Due to the manner in which the amino-acids are linked together. — 

 If to protein solution some strong potash is added, and then a few drops 

 of weak copper sulphate solution, a pretty violet-pink colour is produced ; 

 this colour can be readily distinguished from the bluish shade which would 

 be formed if the protein were absent. 



The percentage composition of dry protein depri\ed of mineral matter 

 is generally given as follows: — 



Carbon ... ... 50.6 



Hydrogen ... ... 6.5 



Nitrogen ... ... 15.0 



S'ulphur ... ... 0.3 



Oxygen ... ••• ^i-5o 

 It will be seen that carbon constitutes more than one-half, 

 about a sixth of the whole. 



Classification of Proteins. 



ALBUMENS. — Albumens are present in blood, milk, eggs, seeds, and 



bioplasm generally. The albumens differ but slightly from one another, 



and are named according to their origin. Thus, lactalbumen is found 



in milk (but in small quantity compared with its chief protein caseinogen), 



