156 PHYSIOLOGY 



solution was homogeneous in that it could be filtered without change,, 

 and could be kept for months without deposition of the gold. The 

 latter was, however, thrown down on addition of mere traces of 

 impurity, though greater stability could be conferred on the solution 

 by adding to it a little ' jelly,' i.e. a weak solution of gelatin. In 

 1899 Bredig showed how similar hydrosols might be prepared from a 

 number of different metals, viz. by the passage of a small arc or 

 electric sparks between metallic terminals submerged in distilled water. 

 If, for example, the terminals be of platinum, the passage of the current 

 is seen to be accompanied by the giving off of brown clouds, which 

 spread into the surrounding fluid. These clouds consist of particles 

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

 the smaller which are ultra-microscopic in size, fc.e.from 5 JULJUL to 40 JULJU* 

 -remain in suspension, 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. 



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

 which a solution of gelatin sets on cooling. It is clear, hyaline, appa- 

 rently structureless, and possesses considerable elasticity, i.e. resist- 

 ance 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 inter- 

 stices of which is embedded the second phase, consisting of a very 

 weak solution of gelatin. If the process be observed under the micro- 

 scope, 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 

 concentrated 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 



* One p is one -thousandth of a millimetre ; one pp. is one-thousandth p, t.e, 

 one-millionth of a millimetre. 



