306 PROTEINS 



plants which are subject to periodic drought possess only 

 small amounts of soluble crystalloids in the cell sap. 



In the case of the barley, it was observed that an exposure 

 for one night to a temperature of 7 C. reduced the yield of 

 soluble proteins by about one-third as compared with a control 

 experiment in which the temperature was not so lowered. This 

 salting out effect is much increased if the cell sap becomes acid 

 on cooling, as is not infrequently the case. 



If the low temperature be long continued, the precipitated 

 proteins will not again enter into solution when the amount of 

 water is increased by raising the temperature ; on the other 

 hand, if the temperature be suddenly raised, the precipitated 

 proteins will re-dissolve, provided that they have not stood too 

 long, and thus the plant will not be greatly harmed. 



B. Chemical Properties. 



I. Precipitation reactions. 



The proteins have both acid and basic properties ; thus, 

 casein may be looked upon as typically acid, seeing that it 

 dissolves in alkalis to form sodium and potassium salts, whilst 

 the histones and protamines are powerful bases. All proteins, 

 however, have basic properties, which enable them to form 

 insoluble salts with a great many of the ordinary alkaloidal 

 reagents, such as phosphotungstic, tannic, picric, ferrocyanic, 

 and trichloro-acetic acids. They are also precipitated by potas- 

 sium ter-iodide (a solution of iodine in potassium iodide) and 

 by the double iodides of potassium with mercury, bismuth, 

 and cadmium. 



The strong mineral acids also precipitate proteins. 



In consequence of this dual nature of proteins they are 

 classed as amphoteric electrolytes (see below). 



The salts of the heavy metals also produce insoluble pre- 

 cipitates with the proteins, a fact which is made use of in the 

 administration of egg albumen as an antidote in cases of poison 

 with salts of the metals. Moreover, the antiseptic action of 

 mercuric chloride is most probably connected with this forma- 

 tion of insoluble salts. 



Amongst the salts most frequently used as precipitants for 

 proteins are the chloride and acetate of iron, the sulphate and 



