THE CONSTRUCTION OF PROTEINS 167 
tion. Nearly all are insoluble in alcohol and ether; they 
all dissolve in strong mineral acids and in caustic alkalies, 
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 the simpler proteins 
have been divided are the following :— 
1. Aupumins.—These are soluble in distilled water, and 
if the solution is heated, the protein is converted into a 
peculiarly insoluble form, known as coagulated protein, and 
deposited as a granular or flocculent precipitate. As the 
temperature rises the liquid becomes markedly opalescent 
before the separation of the protem. 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 protein solution. This 
point is known as the coagulation temperature. Albumins 
can be precipitated unchanged by saturating their solutions 
with sodio-magnesium sulphate. They are not of frequent 
occurrence in plants, but can be extracted from certain 
roots. 
2. GuopuLins.—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 proteins found in plants belong chiefly to this 
class. Globulins can be readily extracted from most seeds, 
