SURFACE ACTION 57 



rise of temperature, in accordance with the general rule. At a temperature of 

 liquid air, charcoal adsorbs gases to such a degree that it is used by Dewar to 

 produce a high vacuum. 



According to Arrhenius (1912, p. 29) adsorption by charcoal of gases which liquefy with 

 difficulty, such as hydrogen and helium, is directly proportional to their pressure at 

 ordinary temperatures, and of all gases (as well as some dissolved substances) at high 

 temperatures. The fact suggests that deviations may be due to something like liquefaction 

 on the surface. Titoff (1910, p. 673), indeed, concludes that ammonia is partially liquefied, 

 because of the rate of increase of heat of adsorption as is approached. 



Since the gas is condensed on the surface, according to some observers even 

 liquefied in certain cases, and when gases are compressed, heat is evolved, it is 

 not surprising to find that adsorption is attended by production of heat. 

 Titoff (1910, p. 658, etc.) has determined this in a number of instances, and 

 his data will be made use of later in discussing the nature of oxy haemoglobin. 

 It is scarcely necessary to remind the reader that this adsorption of gases by 

 surfaces is not a chemical reaction. If oxygen combined with the charcoal 

 used to make a vacuum, so that CO or CO 2 were produced, it is obvious that 

 no disappearance of gas would take place. Moreover, the adsorbed gas can be 

 driven off again by heat. 



The adsorption of water vapour on the surface of vessels which have been 

 dried in a vacuum desiccator is a well-known source of trial to the chemist. 



When finely divided platinum is exposed to a mixture of oxygen and hydrogen, 

 combination takes place between these gases, with the formation of water. 

 Faraday (1844, p. 165) suggested as an explanation of this, phenomenon 

 that a condensation of the gases took place on the surface of the platinum, 

 so that the molecules were brought into close contact. 



It is interesting to note the clear conception of surface condensation which Faraday had 

 formed. On p. 180 of his "Experimental Researches on Electricity" (1839) he speaks of 

 an "attractive force of bodies" causing association more or less close, without at the same 

 time producing chemical combination, but "which occasionally leads, under very favourable 

 circumstances, to the combination of bodies simultaneously subjected to this attraction.' 

 On p. 181 he refers to "the attraction between glass and air, well known to barometer 

 makers," and to the fact that they have no power of combination with each other. On 

 p. 181, again, mention is made of the power of water vapour to condense upon, although not 

 to combine with clay, charcoal, and turf, "assisted a little, perhaps, by a very slight solvent 

 action" in the latter case. (See the present author's letter to Nature, vol. xciv. (1914) p. 253.) 



The question will come up for further discussion in Chapter X. 



II. The Adsorption of Sugar. With respect to the mechanism of the action 

 of enzymes, it is of importance to know whether sugars and related substances 

 are adsorbed. It appears that sugar does not lower the surface tension at 

 the interface between water and another phase to any great extent. It has 

 been shown, however, by Michaelis and Rona (1909, p. 492), and by Parkin 

 (1911, p. 16), that adsorption does occur. The diminution of surface energy 

 must, therefore, concern one or more of the other forms of surface energy 

 which we have referred to. Michaelis and Rona, in fact, suggest that the 

 adsorption may be due to the change of compressibility or of solubility at the 

 interface (see also Wiegner, 1911, p. 126). 



III. Salts. Inorganic salts, although raising surface tension at the air- 

 water interface, lower it at a water-hydrocarbon interface, as Lewis has 

 shown (1909, i. p. 469). Theoretically, then, there is a possibility of adsorption 

 at such an interface. The actual fact can be demonstrated experimentally. 

 J. J. Thomson (1888, p. 192) describes an experiment by Dr Monckman and 

 himself, in which a deep coloured solution of potassium permanganate emerged 

 almost colourless after trickling through finely divided silica. Samec (1911, 

 p. 155) quotes an investigation by Kugel, in which it was found that the 

 apparent solubility of the more insoluble salts might be as much as one thousand 

 times more in starch solutions than in water, owing to adsorption by the starch 

 granules. 



IV. The Nature of Dyeing and Staining. The first stage of this process is 

 almost certainly one of adsorption. How far other processes, such as solid solu- 

 tion and chemical reaction, play a part in later stages, will be discussed presently. 



