METABOLISM 65 



In such a structure substances can diffuse 

 and chemical action can go on as rapidly 

 as in a liquid. Substances which can 

 form structures of this sort are called col- 

 loids (from the Greek word meaning glue). 



A familiar example is gelatin which 

 passes readily from the solid to the liquid 

 state on heating and becomes solid again 

 on cooling. The living substance, how- 

 ever, more closely resembles white of egg 

 which becomes coagulated at higher tem- 

 peratures and suffers a permanent change. 

 Uncoagulated white of egg can be partly 

 solidified by cooling to a low temperature 

 and completely solidified by partial loss 

 of water: it can be liquefied by reversing 

 the process. Protoplasm seems to pass 

 very readily from the solid to the liquid 

 state and vice versa without change of 

 temperature or loss of water. 



In the opinion of some writers proto- 

 plasm is comparable to an emulsion such 

 as mayonnaise salad dressing. Such an 

 emulsion can pass under appropriate con- 

 ditions from a solid state, resembling 



