COLLOIDS AND CRYSTALLOIDS. 373 



nothing to do with the osmotic result, otherwise than as it 

 affects the state of hydration of the septum." 



Prof. Graham has classed various substances according 

 to the degree in which they possess this property of passing 

 when in a state of solution in water, through membrane ; 

 those which pass freely being termed crystalloids, and those 

 which pass with difficulty, colloids. 



This distinction, however, between colloids and crystal- 

 loids which is made the basis of their classification, is 

 by no means the only difference between them. The 

 colloids, besides the absence of power to assume a crystal- 

 line form, are characterised by their inertness as acids or 

 bases, and feebleness in all ordinary chemical relations. 

 Examples of them are found in albumen, gelatin, starch, 

 hydrated alumina, and hydrated silicic acid, etc. ; while 

 the crystalloids are characterised by qualities the reverse of 

 those just mentioned as belonging to colloids. Alcohol, sugar, 

 and ordinary saline substances are examples of crystalloids. 



Absorption by blood-vessels is the consequence of their 

 walls being, like the membranous septum, of the endos- 

 mometer, porous and capable of imbibing fluids, and of 

 the blood being so composed that most fluids will mingle 

 with it. The process of absorption, in an instructive, 

 though very imperfect degree, may be observed in any 

 portion of vascular tissue removed from the body. If such 

 an one be placed in a vessel of water, it will shortly swell, 

 and become heavier and moister, through the quantity of 

 water imbibed or soaked into it ; and if now, the blood 

 contained in any of its vessels be let ou,t, it will be found 

 diluted with water, which has been absorbed by the blood- 

 vessels and mingled with the blood. The water round the 

 piece of tissue, also will become blood-stained ; and if all 

 be kept at perfect rest, the stain derived from the solution 

 of the colouring matter of the blood (together with which 

 chemistry would detect some of the albumen and other 

 parts of the liquor sanguinis) will spread more widely 



