

upon Surface-Films. 371 



arc lamp, I was struck with the contrast between the appear- 

 ance of the two parts. The lower, more aqueous, layer 

 was charged with motes, while the upper, more etherial, layer 

 was almost perfectly free from them. Some years ago I had 

 attempted the elimination of motes by repeated distillation of 

 liquid in vacuum, conducted without actual ebullition, but I 

 had never witnessed as the result of this process anything so 

 clear as the etherial mixture above described. 



The observation with the ternary association, which happened 

 to be the first examined, is interesting, because the approxi- 

 mate equality of the liquids suggests that the explanation has 

 nothing directly to do with gravitation. But the presence of 

 alcohol is not necessary. Ether and water alone shaken 

 together exhibit the same phenomenon. It would appear that 

 when the two liquids are mixed together in a finely divided 

 condition, the motes attach themselves by preference to the 

 more aqueous one, and thus when separation into two distinct 

 layers follows, the motes are all to be found below. 



An obvious explanation, which, however, stands in need of 

 confirmation, is that under the play of the capillary forces the 

 energy is least when the motes, which may be presumed to be 

 denser than either liquid, are in contact with the denser 

 rather than with the rarer of the two. The density here 

 referred to is that which occurs in Laplace's theory of capil- 

 larity, and may need to be distinguished from ordinary 

 mechanical density. 



I have lately endeavoured to obtain some confirmation of 

 the views above expressed by the use of other liquids. It 

 would evidently be satisfactory to exhibit the selection of 

 motes by the upper, instead of by the lower, layer. Experi- 

 ments with bisulphide of carbon and water, and also asso- 

 ciations of these two bodies with alcohol, which acts as a 

 solvent to both, gave no definite result, perhaps in consequence 

 of a tendency to the formation of a solid pellicle at the 

 common surfaces. But with chloroform and water, and with 

 associations of chloroform, water, and acetic acid (acting as a 

 common solvent) , the experiment succeeded. The motes were 

 always collected in the upper, more aqueous, layer, even when 

 the composition of the two layers into which the liquid sepa- 

 rated was so nearly the same that a few additional drops of 

 acetic acid sufficed to prevent separation altogether. 



In this and similar cases a marked tendency to foaming 

 may be observed when the composition is such that separation 

 just fails to take place. 





