26 Elementary Biology. 



A. Proteids and albuminoids. Generally speaking, 

 these are substances of unknown constitution, but with a 

 percentage composition, varying in round numbers 



c H N os 



from 51 7 15 21 -5 



to 55 7-5 16-5 23 5 2 



They are all incapable of being crystallised. They are 

 soluble in water, or at least swell up in it. When treated 

 with nitric acid (HNO 3 ), they take on a yellow colour. They 

 are decomposed by the acid, and the yellow deposit, or 

 precipitate, formed dissolves into an orange-red solution 

 on the addition of a solution of ammonia (NH 3 H.,O). 

 Polarised light is turned to the right when passed through 

 a solution of a proteid. 



Proteids may be divided into two chief classes 

 (a) albumins and globulins, which are distinguished from 

 (/;) albuminates, by the fact that when subjected, in solu- 

 tion, to a temperature of over 73 C. they are coagulated 

 or rendered solid, whilst albuminates are not so affected. 

 Albumins, moreover, are soluble in cold water, whilst 

 globulins are not, unless some neutral salt be added, e.g. 

 common salt (NaCl). It is from these two groups of sub- 

 stances that most of the proteid matter of the animal and 

 vegetal worlds is derived. As examples of animal albumins 

 may be mentioned egg-albumin, or white of egg, and serum- 

 albumin, or the albumin found in the fluid portion of blood, 

 whilst the main constituent of vegetal protoplasm is vegetal 

 albumin. 



Of globulins, myosin, the chief constituent of muscle, 

 and glutin, which enters largely into the composition of 

 seeds, may be taken as examples. 



Casein, the chief constituent of milk in the animal world, 

 and the principal source of nourishment in beans, peas, &c., 

 may be given as illustrating the class of nlbuminates. The 

 albumins become changed into albuminates if treated with 



