212 REPOKTS OX THE STATE OF SCIENCE. 



ascribes this to surface-tension. Quincke ^ has shown that in a system of 

 immiscible liquids A, B, and C, if the sum of the surface- tensions between 

 A — B and B — C is less than that between A — C, B must spread between 

 A and C, forming an intermediate layer. Bechhold shows numerically 

 that this is the case in the system mastic-gektine-water, and that the 

 mastic particles must therefore become coated with gelatine, which acts 

 as a protective colloid ti a mastic suspension. It is obvious that as the 

 felatine- coated particles have greater attraction for water and less for each 

 other as compared with mastic, they will have less tendency to coa'esce. 

 It may also be suggested that the gelatine may form a permanent double 

 layer in the electric sense, which will render the neutralisation of the 

 parti'-les by oppositely charged ions difficult or impossible. 



Platinum and gold sols exert a strong catalytic influence similar to 

 that of platinum black and spongy platinum, decomposing hydrogen 

 peroxide, ctusing combination of oxygen and hydrogen, and other similar 

 effects. These seem merely due to the extended surface and very fine 

 division of the metals, and belong rather to tbe general theory <<f catalysis 

 than to thdt of the colloid state. Metallic sols have, however, been 

 styled ' inorganic ferments ' from the analogy of their action to that of 

 tiie catalases, which are also colloidal, and which bring about similar 

 reactions. Like them the action of the metallic sols is favoured by 

 warmth, and in the case of hydrogen peroxide by small additions 

 of alkali, while larger quantities slow or prevent it. In both cases 

 certain substances, such as hydrogen sulphide and cyanide, exert a 

 i so nous ' or inhibitory efl'ect on the catalyst, which if slight is slowly 

 recovered from. It is doubtful, however, if the resemblance is more than 



analogy. 



The colour of metallic hydrosols is often intense, and varies with their 

 condition, and especially on approaching flocculation ; that of silver from 

 dark brown to brown-violet, changing to deep green before flocculation. 

 Gold hydrosols vary from intense red to blue-violet, taking the latter tint 

 before precipitation by electrolytes, though some blue-gold sols are stable. 

 The change of colour does not depend on the average size of the particles, 

 but apparently on their aggregation. Kirchner and Zsigmondy - found 

 that a red-gold sol rendered stable by gelatine becomes blue on e-vapora- 

 tion to dryness, and under the microscope shows intensely coloured masses 

 of many submicrons in a colourless medium. On moistening or redissolving 

 these become again distributed, and the red colour is restored. Planck -* 

 has shown that the light absorbed by small particles, which act as ' optical 

 resonators ' to the electro-magnetic light-wave, must become redder and 

 that transmitted bluer as they approach each other. Seen in the ultra- 

 microscope the gold particles usually reflect yellow light. 



It has been noted that the precipitated gels of inorganic colloids are, 

 in the opinion of the writer, not identical in structure with the true 

 or'^anic jellies, but at least in many cases they possess strong points of 

 resemblance, and from their fine state of division I'etain many colloidal 

 properties, most marked, of course, when they are reversible. They 

 usually retain a portion of water with great obstinac}-, so that it has often 

 been miste-ken for water of true chemical hydration. In some cases, 

 however, the gels are probably true hydrates, which retain additional 

 water by their surface-attraction. Van Bemmelen, Biitschli, and others 



' 'inn Phy.^. (3), 1888. 35, 580-642. = Jbld. (4), IflOl, 15, GTI^-r.R.o. 



^ Jhiif. (4), 191)0, 1, 09-122. 



