it on 
B,—CHEMISTRY. 67 
where  =surface tension, and p and p denote the density and partial 
pressure of the vapour respectively. Working with the vapour of methyl 
acetate, Iredale found in this way that at a temperature of 26° C. and 
a partial pressure of 62 mm. of mercury, q=4.5 x 10° grm. per square 
centimetre of surface. From this result we can readily calculate that 
there are 0.87 X10'* molecules of methyl acetate adsorbed per square cm., 
and that the area per molecule is 27x107"* sq. cm. As under the condi- 
tions corresponding to this calculation the molecular surface layer was 
probably not quite saturated (in the unimolecular sense), we may expect 
the value found to be of the same order of magnitude but somewhat 
greater tlian the values found by Adam for the cross section of the head 
group of the higher saturated fatty acids (25x 10~'") and of the esters 
(22x10-"* for ethyl palmitate and ethyl behenate). We may, therefore, 
say that Iredale’s results appear to indicate the formation of a primary 
— unimolecular layer built up by adsorption from the vapour phase. 
the surfaces of liquids 
in a liquid concentrates preferentially at the liquid-air or liquid-vapour 
Langmuir has measured the adsorption of a number of gases ab 
low temperatures and pressures on measured surfaces of mica and glass, 
and has arrived at the conclusion that the maximum quantities adsorbed 
are always somewhat less than the amounts to be expected 1n unimole- 
cular surface layer. EK. K. Carver, who has measured the adsorption of 
toluene vapour on known glass surfaces, has arrived at a similar con- 
clusion. The view that the maximum adsorption from the gas phase 
cannot exceed a unimolecular layer has, however, been much criticised. 
Thus, for example, M. E. Evans and H. J. George, on the basis of 
their own measurements on the adsorption of gases on a known surface 
of glass wool, combined with the data obtained by Miilfarth, have con- 
cluded that the adsorption layer may be several (and in some cases: 
many) molecules thick. 1. may well be that the formation of a uni- 
molecular ‘ saturation * layer only occurs in the case of molecules with: 
relatively very active atoms or atomic groups, whose strong localised! 
fields of force suffice to produce powerful attraction and orientatiom 
and an almost complete saturation of the ‘ stray’ fields of the surface 
molecules of the adsorbing surface, especially when the thermal tem- 
perature agitation is sufficiently small. In the case of molecules with 
weaker or more symmetrical fields of force, there may be relatively 
little orientation, and an extension of the attraction field of the adsor- 
bent through layers of the adsorbate many molecules thick. It would 
be rash to theorise too much on this subject until more data are accu- 
mulated, but it may be pointed out that in his investigations on the 
spreading of surface films and on the theory of lubrication, Hardy has 
been led to distinguish between primary spreading (primary unimole- 
cular films) and secondary spreading (secondary relatively thick sheets). 
Let us now consider another type of formation of surface layers at 
namely, the case where a substance dissolved 
interface. Gibbs, and later J. J. Thomson, have shown that if a 
dissolved substance (in relatively dilute solution) lowers the surface 
tension, it wili concentrate at the surface. That such a phenomenon 
actually occurs has been qualitatively demonstrated in the experiments 
of D. H. Hall, J. von Zawidski, and F. B. Kenrick and C. Benson, 
