APPEARANCE OF THE FOAM-DROPS 21 



stood two months in the sun, showed themselves quite abnormal 

 in the fact that they were very transparent ; 'for their investiga- 

 tion no clearing up with glycerine was requisite. By remaining 

 longer in the weak solution of KjCOg under the cover-slip, these 

 froths were in like manner relatively quickly attacked and de- 

 stroyed, while those obtained from moderately thickened oil could 

 remain without harm in the solution for many days. 



As I mentioned above, the extremely minute drops of oil that 

 are split off become frothy immediately. This fact naturally leads 

 one to suppose that the production of froth can also be brought 

 about by placing an oil-drop in a solution of K^COg. Experiments 

 proved successful, but slowly, with more concentrated solutions 

 of KgCOg, although with a moderately strong solution the 

 drop was finally changed after some days into an even, fine 

 froth. A much more energetic formation of froth goes on in 

 solutions of from 1 to 2| per cent. The drops when brought 

 into the solution become milk-white at once, and a circulatory 

 streaming is set up, similar to that which was described earlier 

 (see above, p. 14) for the oil-drops treated with white of egg 

 or soap. The edge of the drop gradually becomes darker and more 

 opaque, and sometimes retort-shaped processes project from the 

 surface, in which the very peculiar relations of the currents are 

 difiicult to understand. By continued growth of the dark margin 

 of the drop, the whole drop finally becomes opaque. After twenty- 

 four to forty-eight hours the formation of the froth is completed ; 

 the froth is very fine and regular. Nevertheless, drops of oil- 

 lather obtained in such a manner proved from various experiments 

 unsuitable for the observation of the phenomena of streaming, 

 for which reason I did not continue to employ this method of 

 preparing foam. I should be inclined to think, however, that with 

 more elaboration it might develop into a very simple and good 

 method. It is interesting to note that the oil-drops which have 

 stood a longer time in a more concentrated solution of KjCOj, 

 without having formed good froths, after being transferred to 

 water develop quickly into good froths, which harmonises well 

 with the explanation of froth production given above. 



Successfully prepared drops of oil-lather are, as remarked, 

 completely milk-white in reflected light, and, if at all thick, 

 quite opaque in transmitted light; smaller drops, on the 

 other hand, or larger ones pressed out into a thin layer, 

 appear brownish yellow in transmitted Ught. The frothy 

 structure can be studied without further trouble in the 

 smaller drops that split off, since they are sufficiently trans- 



