THE CHEMISTRY OF THE CELL 7 



the cell consists of an envelope which can be described as 

 semi-permeable, that is, permeable to one class of bodies but 

 not to another. The contents of the envelope 

 consist of liquid plasma or sap, throughout 

 which, and lining the interior of the envelope, 

 is a semi-fluid mucilaginous substance referred 

 to generally as protoplasm. This is in a con- 

 tinual state of movement and of chemical 

 change ; and in the midst of it is a cell 

 nucleus. 



The substances entering or leaving the cell 

 must obviously be possessed of certain physi- 

 cal properties if they are to pass through 

 the semi-permeable membrane. It is necessary, therefore, to 

 consider the different conditions which the matter composing 

 the various substances entering and leaving the cell labora- 

 tory may assume. There are first solid insoluble bodies ; 

 these, of course, are not likely to pass through the cell mem- 

 brane ; on the other hand, substances in true solution, such as, 

 e.g., salt dissolved in water, will as a rule pass freely through. 

 There are, however, intermediate conditions in which matter 

 can exist. 



Colloids. It was first shown by Graham that by appro- 

 priate means solutions could be obtained, which, while devoid 

 of visible particles, were incapable of passing unchanged 

 through a parchment membrane. Substances which were 

 soluble and which would pass while in solution through a 

 parchment membrane Graham termed crystalloids ; substances 

 which while soluble as judged by ordinary physical tests would 

 yet not pass through a parchment membrane he termed 

 colloids. A typical case illustrating the difference between a 

 colloid and a crystalloid is the one selected by Graham, viz., 

 silicate of soda. If a dilute solution of silicate of soda is 

 carefully acidified with hydrochloric acid, no precipitation 



