THE PROPERTIES OF COLLOIDS 139 



of all sizes. The larger settle at the bottom of the vessel, the smaller 

 which are ultra-microscopic in size, i.e. from 5 /X/A to 40^* remain in sus- 

 pension, and we obtain a brown fluid which can be filtered through paper 

 or even through a Berkefeld filter without losing its colour. It may be 

 kept for months without any deposit taking place. The addition of minute 

 traces of electrolytes precipitates the platinum particles, leaving a colourless 

 fluid. We shall have to return later on to the consideration of the behaviour 

 of these metallic sols. 



Colloidal solutions or sols may be divided into two classes, emulsoids 

 and suspensoids, according as they may be regarded as suspensions of liquid 

 in liquid or as suspensions of solid particles. 



Most protein solutions are emulsoids, while the metallic sols belong to 

 the class of suspensoids. Dilute egg-white is an emulsoid, but if it be boiled, 

 although no visible precipitation is produced, the fine particles are coagulated 

 and it behaves as a suspensoid. 



PROPERTIES OF GELS. A typical hydrogel is the firm mass in which 

 a solution of gelatin sets on cooling. It is clear, hyaline, apparently structure- 

 less, and possesses considerable elasticity, i.e. resistance to deforming force. 

 It may be regarded as formed by the separation of the warm pseudo-solution 

 of gelatin into two phases : first a solid phase, rich in gelatin and forming 

 a tissue or meshwork, in the interstices of which is embedded the second 

 phase, consisting of a very weak solution of gelatin. 



If the process be observed under the microscope, according to Hardy minute drops 

 first appear which, as they enlarge, touch one another and form networks. In stronger 

 solutions the first structures to make their appearance consist, not of the more con- 

 centrated phase, but of droplets of the dilute solution of gelatin ; the stronger solution 

 collects round these drops and solidifies to a honeycomb structure. 



In many cases the more fluid part of the gel is practically pure water. 

 In such a case immersion in alcohol causes a diffusion outwards of the water, 

 which is replaced by alcohol with the formation of an alcogel. In a dry 

 atmosphere the gel loses water and becomes shrivelled and dry, but in 

 some cases, e.g. gelafin, it can resume its former size and characters on 

 immersion in water. Other gels, such as silicic acid or ferric hydrate, lose 

 the power of swelling up after drying. The change in them is therefore 

 irreversible. A gel adheres to the last traces of water with extreme tenacity. 

 In consequence of its structure, it presents an enormous extent of surface 

 on which adsorption can take place. At this surface the vapour-tension of 

 fluids is diminished, as well as the osmotic pressure of dissolved substances. 

 On this account gelatin must be heated for many hours at a temperature 

 of 120 C. in order to be thoroughly dried. When dry, it, as well as other 

 solid colloids, can exert a considerable amount of energy when caused to 

 swell up by moistening. This energy was made use of by the ancient 

 Egyptians in the quarrying of their stone blocks by the insertion of wedges 



million 



* One /A is one-thousandth of a millimetre ; one /A/A is one-thousandth /A, i.e. one- 

 th of a millimetre. 



