THE PROPERTIES OP COLLOIDS , , , 



which are ultra-microscopic in size, i.e. from 5 MM to 40 MM*-r.M,ni,, 

 pension, and we obtain a brown fluid which can be filter,,! ,1,,,,,',,-h 

 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 colourlesg 

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

 of these metallic sols. 



PROPERTIES OF GELS. A typical hydrogel is the firm mass in whic-h 

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

 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 alco-gel. In a dry 

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

 some cases, e.g. gelatin, 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 

 of wood ; water was poured on the wood, and the swelling of the wedges 

 split the rock in the desired direction. | 



On account of the extent of surface it is practically impossible to wash 

 out the inorganic constituents from a gel. The diminution of the osmotic 

 pressure of many dissolved substances at surfaces causes the concentrati. 

 at the surface of the solid phase to be greater than that in the raw 

 medium. Thus, if dry gelatin be immersed in a salt solution il 



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

 millionth of a millimetre. 



t According to Rodewald, the maximal pressure with which dry s 

 water amounts to 2073 kilo, per sq. cm. 



