32 NATURAL SCIENGE. Jan., 
oil must be kept in a watch glass, at a temperature of from 50—60° 
Centigrade, for twelve days, or at 80° C. for about three days. It 
becomes nearly colourless, and rather thicker. This is placed in a 
mortar, and powdered, but not anhydrous, carbonate of potash is 
rubbed into it. Although the desired result is the formation of a 
soap in the oil, good froths are not obtained by adding fatty acids or 
soaps to the oil. 
The prepared froths, then, consist of a multitude of tiny droplets 
of a solution of soap in water, each droplet being surrounded by a 
film of liquid oil. The films, when they are in contact, naturally run 
together, and so a structure of polyhedral figures is built up, just as 
when one blows into soap solution in a saucer, the mass of bubbles can 
be seen to consist of polygonal figures. Minute examination of the froth- 
structure shows that at the nodes where the films meet, owing to their 
liquid condition, slight expansions occur (of. cit., p. 20, Fig. 2). These 
expansions, when the bubbles are very small, give a granular appear- 
ance to the whole structure—an appearance which is more striking 
whena part of the froth more than one layer of bubbles thick isexamined. 
The general optical effect is that of a meshwork, with thickenings at 

Fic. 1.—Foam under high magnification showing reticulate and fibrillar appearance 
and nodes. 
the nodes. Biitschli found that when adventitious particles as, for 
instance, of lamp-black, are mixed with froth, these particles tend to 
collect at the nodes; but a careful study of the froth shows that node 
thickenings alone are enough to account for the granular appearance 
of protoplasm. He also shows how the familiar fibrillar, reticulate, 
and lamellar appearances, on careful focussing, are resolved into 
arrangements of bubbles. 
He shows and explains, theoretically, how surface tensions make 
drops of such a froth, suspended in a liquid, assume a globular form, 
save under the influence of special internal or external forces. For 
some not very clear reason, the larger bubbles come to group them- 
selves in the interior of each drop. Round the periphery, where the 
surface tensions are similar, a single row of bubbles of equal size 
becomes arranged (Fig. 1). The films separating these bubbles are 
radially arranged, and so there is formed a definite alveolar border. 
When the bubbles are small, this looks like a striated border. A 
similar border is naturally formed round any internal large vacuole. 
The alveolar layer, spite its definite appearance, remains as liquid as 
the rest of the foam, and persists while the drop is flowing. 
