THE CONSTEUCTION OF PEOTEIDS 175 



but they are decomposed during the process. Their 

 solutions have generally a power of deflecting a ray of 

 polarised light to the left. 



The best known groups into which proteids have been 

 divided are the following : 



1. ALBUMINS. These are soluble in distilled water, and 

 if the solution is heated, the proteid is converted into a 

 peculiarly insoluble form, known as coagulated proteid, and 

 deposited as a granular or flocculent precipitate. As the 

 temperature rises the liquid becomes markedly opalescent 

 before the separation of the proteid. The change takes 

 place at a point which lies between 65 and 80 C., its 

 exact place depending upon the nature of the albumin and 

 the reaction and concentration of the proteid solution. 

 This point is' known as the coagulation temperature. 

 Albumins can be precipitated unchanged by saturating 

 their solutions with sodio-magnesic sulphate. They are 

 not of frequent occurrence in plants, but can be extracted 

 from certain roots. 



2. GLOBULINS. These differ from albumins in not 

 being soluble in distilled water. They can be dissolved by 

 adding a little neutral salt, such as sodium chloride. Their 

 solutions are coagulated on heating, but they show a con- 

 siderable variability as to the coagulation temperature, 

 which in the case of some is as low as 55 C. Most of 

 them, however, remain unchanged below 75-80 C. They 

 can be precipitated by saturating their solutions with 

 magnesium sulphate. If sodium chloride is used instead 

 of the latter, an incomplete precipitation usually takes 

 place. Different members of the group show different 

 degrees of solubility in solutions of sodium chloride ; some 

 require only a trace of the salt ; others need 8-10 per cent.; 

 and a few are soluble only in saturated solutions. 



The proteids found in plants belong chiefly to this 

 class. Globulins can be readily extracted from most seeds, 

 and probably this form of proteid is the one which occurs 

 in the green parts of plants. 



