CRYSTALLOIDS. 27 



called crystalloids and colloids, dissolved in water, in the lipoids, 

 or in each other, and surrounded by a membrane which is per- 

 meable towards certain substances but not towards others (semi- 

 permeable, as it is called). On a larger scale, the same general 

 conditions exist in all of the animal fluids, such as the blood, the 

 lymph, the secretions and the excretions, so that we may study 

 the laws with a view to applying them to both cells and body 

 fluids. 



Properties of Crystalloids. As their name implies, these 

 form crystals under suitable conditions. When present in solu- 

 tion they diffuse quickly throughout the solution, and can readily 



Fig. 1. Dialyser made of tube of parchment paper suspended in a vessel 

 of distilled water. The fluid to be dialysed is placed in the tube, and the 

 distilled water must be frequently changed. 



pass through membranes, such as a piece of parchment, placed 

 between the solution containing them and another solution. This 

 process is called dialysis, and the apparatus used for observing 

 it, a dialyser (see Fig. 1). Dialysis differs from filtration, the 

 latter process consisting in the passage of fluids, and the sub- 

 stances dissolved in them, through more or less pervious mem- 

 branes as a result of differences of pressure on the two sides of 

 the membrane. If instead of using a simple membrane, such as 

 parchment, we choose one which does not permit the crystalloid 

 itself to diffuse, but permits the solvent to do so a semipcrmeablc 

 membrane, as it is called, a very interesting property of dis- 

 solved crystalloids comes to light, namely, their tendency to ex- 



