370 
Actinozoa. These “are distinct from the Spongida, 
although some synthetic-minded morphologists classify 
all together as Ccelenterata.”” In treating of the freshwater 
Hydra we notice the “old story” repeated that “if the 
body be turned inside out, the old ectoderm [why the 
adjective ?] takes on the digestive power and the former 
endoderm that [takes on the function] of the skin.” 
The order Hydrocorallina is placed as the last of the 
Hydrozoa, with the families Milleporide and Stylasteride, 
as indicated by Moseley, to whose researches and those 
of Agassiz we are indebted for all we know about the 
order. Mzllepora alcicornis was obtained by Moseley at 
Bermuda. The calcareous tissue of the coral is very hard 
and compact, and the polyps are extremely small. It is 
very difficult to prevail on the polyps to protrude them- 
selves from their cells, but Mr. J. Murray, of the Challenger 
Expedition, succeeded in procuring them in this state on 
two occasions, and the accompanying drawing (Fig. 2) of 
one of the expanded polyps, and of five of its surrounding 
zooids, is from Mr. Moseley’s memoir on the structure of 
this genus. In the centre is seen the short polyp form 
provided with a mouth and with only four short knobby 
tentacles, while grouped a-ound are the five polyps without 
mouths, and for the sake of letting the central zooid be 
clearly seen a sixth mouthless zooid is omitted from the 
sketch ; these latter zooids have from five to twenty ten- 
tacles ; they are much more active than the mouth-bearer, 
and do the work of catching food for it. When alarmed 
all disappear within the framework. 
The article on the group of the Sponges is excellent. 
The author now regards the sponges as forming a sepa- 
rate class independent of the Coelenterata, and situated 
at the very bottom of the Metazoic sub-kingdom, and 
gives a brief sketch of the orders and sub-orders. 
The figures of sponge structure are refreshingly new, 
many of them being from quite recent sources—-such as 
the memoirs of Haeckel, Schulze, and Prof. Sollas himself. 
The beautiful sponge belonging to the genus Euplectella, 
now known to live anchored in the mud in deep seas, or 
attached to the hard bottoms of shallower waters, has had 
its structure ably described by Prof. Schulze, from whose 
memoir the annexed woodcut (Fig. 3) istaken. ZThe mem- 
branous wall—very delicate and thin—which surrounds 
the skeleton is furnished with smooth-edged roundish 
pores of different sizes, irregularly arranged, and varying 
very much in number. These form an open communica- 
tion between the cavities of the chambers and the duct- 
like spaces surrounding them, which penetrate every where 
between the ciliated chambers and extend even to their 
mouths, where they terminate on a tougher membrane, 
which binds together and connects laterally the chamber 
walls. The figure shows the outer portion of a thin 
section taken perpendicularly to the outer surface through 
the side wall of a ridge, and is magnified X 150. Several 
of the ciliated chambers are seen. 
Although the Rhizopods. are described as standing 
“first in the scale of animal organisation,” we find them 
treated of in a chapter before that relating to the 
Infusoria, and we are told in the same paragraph that 
“‘they have ina great degree the same simple constitution 
as several other kinds of animalcules which are grouped 
by naturalists as Protozoa.” We venture to think that 
such a description will be apt to lead the general reader 
astray ; nor was it quite fair of the Editor to allow the 
writer of the article on the Rhizopods to go somewhat 
out of his way in his forty-ninth paragraph to give a view 
of the organisation of the Sponges which will be apt to 
puzzle the reader who has perused the more accurate 
account of the sponge structure given by Prof. Sollas. 
As an example of the beautiful illustrations of the 
Infusoria, which are for the most part taken from Saville 
Kent's excellently illustrated Manual of Infusoria, we give 
the woodcut of Aiipidodendron splendidum. There are 
ew workers with the microscope who devote themselves 
NATURE 
[feb. 15, 1883 
to the study of the Infusoria but must be familiar with 
the stems of that group of animalcules, which gravitate 
about the well known Azthophysa vegetans of Miller ; the 
attached colony stocks putting one in mind of some 
minute fucoid stem. Of this group the species figured 
after Stein is one of the most remarkable, originally 
described and most beautifully illustrated in Prof. Stein’s 
great work. This freshwater form has apparently not yet 
been found in this country, but a nearly allied species, 
k. Huxleyz, has been met with in South Devon. The 
figure shows the compound colony stock at A, the quite 
young colony stock at B, which latter was built up by a 
single monad, which divided by longitudinal fission, pro- 
ducing two parallel, or nearly so, tubes, and one of these 
monads is seen at C free, without a tube. 
In congratulating the Editor on the successful termina- 
tion of his labours, we are not unmindful of the difficulties 
he has had to encounter in trying to secure a more or less 
uniform style of treatment of subjects so varied as 
the different classes and sub-divisions of the animal 
kingdom. 
THE CONDENSATION OF LIQUID FILMS ON 
WETTED SOLIDS 
Ie Poggendorf’’s Annalen for 1877,and in the Phzlo- 
sophical Magazine for 1880,1 have recorded some 
facts which are satisfactorily explicable only on the sup- 
position that the liquid in contact with the glass under- 
goes condensation upon the surface of the latter. In the 
latter paper I was able to show that this condensed film 
visibly altered the resistance experienced by the liquid 
in flowing through the tube. In the paper in the Poggez- 
dorff’ s Annalen it was shown that a difference of poten- 
tial was set up between tke two ends of a capillary tube 
through which water was forced, and that the effect of 
leaving the water in contact with the tube was that this 
difference of potential rapidly diminished. No doubt 
this finds its explanation in the effect of the condensation 
of the liquid on the sides of the capillary tube, causing 
the friction of the water against the tube to become less 
and less, whilst the friction of the water upon the con- 
densed water-film becomes progressively greater, as the 
latter adheres more strongly to the glass. Probably 
simple drying would suffice to restore to the tube the 
originally observed difference of potential between its 
ends. 
Whilst working upon this subject I noticed the large 
E.M.F. produced by a small air-bubble slowly ascending 
through the vertical capillary tube which was full of 
water (see Dr. Dorn, Ann. d. Phys. u. Chem. 1880, S. 
73). At the time I could not account for this, but not 
long ago I constructed an apparatus which allowed of 
alternate drops of water and bubbles of air being driven 
through the capillary tube. This produced a very large 
E.M.F. Probably this increase in the E.M.F, is depen- 
dent upon (a) the increased electrical resistance conse- 
quent upon breaking up the water in the tube into drops 
separated by air-bubbles, and (8) upon an increased 
disturbance of the liquid film adhering to the glass. 
Experimentally these etfects, (a) and (8), might be sepa- 
rated by substituting for water a (practically) perfectly 
insulating liquid. 
Another and very interesting illustration of a liquid 
condensed on the surface of a solid is probably to be 
seen in the familiar fact that water will not clean a greasy 
sheet of glass. ‘ 
As is well known to all workers on surface tension, 
almost the only way of getting a physically clean sur- 
face of glass is by heating the glass in concentrated 
sulphuric acid, to which a little nitric acid has been 
added, and then heating, after washing in pure water 
to remove the acid. Such a glass surface exposed 
to the air for a short time is generally imperfectly 
