160 
POPULAE SCIEl^CE WEWS. 
[November, 1891. 
[Original In POPULAB SCIENCE NEWS.] 
BUBBJ>ES AND FOAM. 
BY J. LAWTON WILLIAMS. 
The bubtiles of itnprisoued air seen floating ou 
. the surface of disturbed water, and their final 
product, foam, iDroceed along quite definite and 
fixed lines of development. Their sizes present 
great difTerences at the time of their formation, 
and they continue to undergo incessant changes 
down to the period of their final disintegration. 
The sizes of bubbles in all liquids, so far as ob- 
served, vary as the intensity of the commotion 
which pi'oduces them. This is true up to a cer- 
tain point, but the maximum of size is speedily 
reached, and, no matter how great the commotion 
beyond that point, larger bubbles will not be 
formed. Although the sizes of bubbles do not 
readily admit of exact measurements, yet there 
seems to be good reason for believing that they 
vary with the temperature of any given liquid, 
and in different liquids at the same temperature. 
Hot water seems to be more favorable to the for- 
mation of large bubbles than cold, and they per- 
sist longer. I believe that cold water, on the 
other hand, is more favorable to the production 
of large crests and masses of foam. Such masses 
seem to be much more abundant at the bases of 
waterfalls in winter than in summer. This fact 
may, however, be due to the greater volume of 
water, and the more favorable conditions for the 
generation of foam presented by icicles and irreg- 
ular surfaces of broken ice. The larger size and 
longer duration of bubbles in hot water is perhaps 
to be explained in part by the diminished surface 
tension resulting from a lighter specific gravity, 
though our ignorance of thermal hydrostatics is 
too profound to sanction any authoritative state- 
ments. That bubbles have different maximum 
sizes and periods of duration in different liquids is 
familiarly shown in water which has been ren- 
dered viscous with soap. The bubbles are much 
larger and last longer than in pure water. Even 
the hard waters of wells cannot be chafed into 
foam of any appreciable duration, no matter how 
vigorously they are beaten; while river water 
laden with surface impurities forms even in the 
smaller rapids flakes of foam as large as a silver 
dollar. 
The genesis of foam from bubbles is a very in- 
teresting process. It may be beautifully illus- 
trated by the following simple experiment : Place 
a little warm water in a basin and rapidly stir it 
with a cake of soap until a suds is fonned. 'I'he 
following facts may then be observed : The suds 
when first formed consists for the most part of 
small vesicles, interspersed here and there with 
large bubbles. These last are quite evanescent, 
bursting by the rupture of contiguous smaller 
ones, producing a half crackling, suppressed roar, 
somewhat like the sound of distant breakers. 
There is a zone of very small vesicles where the 
suds comes in contact with the basin, and others 
of intermediate size interspersed over the surface. 
The sizes 'vary all the w-ay from that of a large 
cufi'-button to the point of a pin. If left undis- 
turbed the larger sizes collapse one by one until 
the whole mass presents quite a uniform aspect. 
If, now, this be vigorously re-stirred, new bub- 
bles, larger than the largest of the first stirring, 
are formed, but they exist only a few seconds at 
longest. Their brief existence is probably due to 
the greater tennuity of the film. I have observed 
suds, however, in which such bubbles persisted 
for several minutes. By the rapid collapse of the 
larger bubbles a uniform size is soon attained 
once more. Repeated stirrings will produce fewer 
large bubbles each time, and finally the most vig- 
orous chafing will produce none at all. Mean- 
time, the surging noise has also subsided. The 
whole mass has a fleecy whiteness ; it has reached 
a condition-of partial equilibrium ; in other words, 
it is foam, and will continue to be so until it 
evaporates and disappears. 
Foam, then, is a direct product of the continu- 
ous subdivision of bubbles, and is formed in nature 
by the chafing of water against resisting surfaces. 
A process similar to the one just described is em- 
ployed in making cake-frosting. In this case the 
resulting froth consists of minute vesicles of albu- 
men. Not all foam, however, is produced by fric- 
tion. In the evolution of gases in alcoholic fer- 
mentations, and in the froth of soda-water pro- 
duced by the escape of carbon dioxide, there is no 
visible friction concerned. It would seem that 
the bursting of bubbles is due in part to evapora- 
tion. Movements and tremors are also visible on 
their inner surfaces before collapse, as if subject 
to intense strains fi'om within. Doubtless in dis- 
turbed water and near resisting surfaces there are 
tangential pulls which hasten the rupture. This 
is rendered all the more probable by the existence 
of a zone of small vesicles (in the experiment) 
next to the sides of the vessel, where the reaction 
between the elastic, shrinking mass and the in- 
elastic vessel is most intense. In the case of soli- 
tary bubbles, probably the energy of surface ten- 
sion, combined with a slight excess of pressure of 
the imprisoned air, hastens the collapse. In ag- 
gregate bubbles, as in river foam, there are a 
number of causes which doubtless conspire to 
effect their dissipation. Certain it is that such 
aggregates move down stream a little way only, 
and then disappear. Generally there are two or 
three bubbles of considerable size in the center of 
the foam-flakes, and these are quite liable to burst 
and produce the instantaneous disintegration of 
all, when the flakes gyrate found and come to rest 
in a quiet part of the stream. This is the most 
common mode of disappearance; but more fre- 
quently the foam coming down in irregular 
patches acquires a rotary movement, and the 
peripheral vesicles moving more rapidly than the 
central ones are either thrown oft" by centrifugal 
impulses, or wheel around upon the center and 
produce mutual destructions. Still another cause 
of dissipation is the collision of two flakes, gen- 
erally resulting only in a destruction of peripheral 
vesicles, and giving birth to two or more new in- 
dependent aggregates. Oftentimes quite large 
flakes of foam will ride over several considerable 
ripples unchanged, and will suffer Instantaneous 
rupture in smooth and quiet water. 
Cataracts, riffs, billows, and breakers all form 
bubbles and ultimately foam. In all cases except 
the first they are formed by the backward move- 
ment of a crest or spray which entraps the air. 
The existence of all bubbles in free water is of 
exceedingly brief duration. It may, however, be 
prolonged by various devices. Bubbles formed 
on water, the surface of which has been covered 
by a film of petroleum, often exist for an indefi- 
nite period. The reason seems to be that the 
viscid oil abolishes the energy of surface tension, 
which we have seen to be such au important 
factor in the rupture of bubbles. Bubbles pro- 
duced by the respiration of aquatic animals, or 
any other evolution of foreign gases, generally 
rise as a series of ebullitions, several forming at a 
time and moving radially from the center of dis- 
turbance. The bubbles ascend so rapidly from 
below that their reflecting surfaces appear like 
glistening rows of pearl beads or superimposed 
hemispheres. Very different in^origin, ^though 
similar in appearance, are the columns of effer- 
vescence which rise in boiling water. They are 
simply waves of rarefied water which ascend to 
the surface. Foam may, however, form in this 
way by the air becoming entrapped at the surface, 
as it does on sap-vats when intense heat is applied. 
SOME CANADIAN BIRDS AND INSECTS. 
NOTES OF THE YEAB 1890 HEAD BY F. B. CAUL- 
FIELD BEFORE MONTREAL CHAI'TER, NO. 3!).5, 
OF THE AGASSIZ ASSOCIATION. 
I REGRET to say that my opportunities for field 
work during the past summer were few indeed. 
I have, therefore, included birds, as well as in- 
sects, in my notes of the season's work. 
On the fourth of April I went down to Iloche- 
lago, my principal object being to visit a grove of 
pine trees in the hope of pi-ocuring some crossbills, 
which are occasionally to be met with there, but 
found none, and indeed but very little life of any 
kind. In a field on the high ground, just east dl 
the Canadian Pacific Railway track, I obtained 
a specimen of the prairie horned lark ( Otocoris al- 
pestris pi-atincola), and still further to the east 
a few song sparrows {Melospiza fasciata) were ol)- 
served in sheltered sunny corners ; these appeared 
to have just arrived, and seemed quite timid, 
skulking in the brushwood, occasionally singing a 
few weak notes. These, with a small fiock of 
bluebirds {Sialia sialis) and a few crows (Corvus 
Americanus), were the only birds observed. No 
insects of any kind were seen. 
On May 24 the flelds and woods in the vicinity 
of the aqueduct were visited. In the reeds grow- 
ing in a large pond at Cote St. Paul the long- 
billed marsh wren (Cistothorus palustrie) was 
heard, and several pairs of red-winged blackbirds 
(Ageloeius phcBtiiceus) flew up and scolded us vigor- 
ously for trespassing on their domain. In a grove 
of elm trees the Baltimore oriole {Icterus ijalhula) 
displayed his brilliant dress of orange and black, 
and the woods resounded with the loud call of the 
golden-winged woodpecker (Colaptes auratus). 
In the meadows, the bobolink {Dolichonyx oryn-. 
vorous), in his wedding dress of black and yellow, 
enlivened the scene with his merry ringing notes, 
and from every shrub and tree, song sparrows 
{Melospiza fasciata) and yellow birds (Dendroica 
estiva) answered him that summer, beautiful sum- 
mer, with its sunshine and flowers, had come. 
Along the aqueduct a few spotted sandpipers 
{Actitis macularia) were seen, and from his van- 
tage ground on the top of. a telegraph pole, a 
kingfisher {Ceryle alcyon) sounded his loud, rat- 
tling call; inileed, many kinds of birds were seen, 
some being quit« abundant. Insects were by no 
means so numerous; hybernated specimens of the 
antiopa butterfly ( Vanessa antiopa) were seen, 
and in dry fields the conmon grass moth {Dlas- 
teria erecthea) was as usual abundant, but little 
else was met with; even the mosquitoes, always 
on hand to receive visitors, appeared in small 
numbers and were not nearly as attentive as ou 
other occasions. 
On the seventh of June the Natural History So- 
ciety held its annual field-day at Lachute, but the 
weather being cool and cloudy the insects were 
again "conspicuous by their absence"'; the most 
interesting species t.aken was a fine saw-fly, I 
think a species of Nematvs, taken by Mr. Fletcher. 
We also found a curious variety (Chrysomela 
philaddphica), a beetle generally common on wil- 
lows and alders; the majority of the specimens 
taken were of a rusty brown color, the common 
or typical form being silvery white, with dark 
green spots and dashes. Butterflies were very 
