214 
The other possible cause of separation would be that 
the times of the tide produced by the sun’s attraction 
coincided with the period of gravitational oscillation 
of the mass of the spheroid; that is, that the period 
of free oscillation would be the same as that of the 
forced oscillation due to the solar tide. After this had 
gone on for some while, the tidal protuberances would 
become so large that, in the opinion of Sir G. H. 
Darwin, one (or both) might break away. He con- 
sidered that the rate of rotation in this case would 
have been about one revolution in five hours. And it 
was this rate that I assumed in my letter to NATURE. 
By the time the earth’s rotation had been reduced so 
far as this, it does not seem impossible that a crust 
~ might have been formed. 
If the material which now constitutes the moon was 
in any way detached from the earth, the matter so 
detached cannot have coalesced into a single sphere 
until Roche’s limit was passed, which would be 2-44 
of the earth’s radii from its centre. During this 
initial stage of the moon’s existence the nearest 
analogy seems to be found in Saturn’s rings. But 
the difficulty remains why the matter detached should 
not have fallen back again. 
As regards the formation of mountains by the con- 
traction of the earth, I have discussed the question to 
the best of my ability in my ‘‘ Physics of the Earth’s 
Crust,’”’ and have come to the conclusion that the 
theory is untenable. O. FIsHER. 
Graveley, Huntingdon, April 4. 
Movements on Water Surfaces. 
I HAVE often wondered what is the real explanation 
of the following observation. If on a bath of soapy 
water a skin is allowed to collect on the surface, and 
then lumps of soapy lather be allowed to drop from 
one’s hands on to it, the skin will crack in all direc- 
tions, radiating from the point where the skin was first 
pierced, and this will continue for some time after the 
initial cause of the disturbance has ceased. The 
phenomenon is very striking, and can be repeated 
several times, after which the effect cannot be pro- 
duced. Also if a cake of wet soap be placed on a 
wet level surface, the moisture is repelled from the 
cake, until the latter becomes surrounded by a dry 
patch. These seem to suggest repulsion of similarly 
electrified bodies. EpwarD A. MartIN. 
Mr. MartiIn’s second question is more easily 
answered than his first. The surface tension of clean 
water is about three times as great as that of water 
containing soap, so when the soap touches the wet 
surface the surrounding wet being no longer pulled 
towards the soap as strongly as it is pulled away, 
obeys the latter force as fast as it can. 
I am inclined to think that his first observation may 
relate to a similar phenomenon, but of this I am not 
sure. With oleate of soda a very small quantity re- 
duces the tension to the lower limit at once, and with 
this I do not think the experiment would succeed. I 
can only suppose, but I do not know it as a fact, that 
with the soap used first a scum is formed, and then 
when fresh lather breaks this at a point there is rather 
less surface tension at this point than there is in the 
surrounding surface. If so, the result observed would 
naturally follow. I should not, however, have: ex- 
pected to find this difference in the surface tensions. 
Or, possibly, the lather from the hand is warmer and 
for this reason has somewhat less surface tension. 
I have often shown in a striking way the diminution 
of surface tension with rise of temperature by holding 
the hand steadily against one side of the rainbow cup 
when the film upon it is already thin and highly 
NG.1.2322,4ViOlO 2) 
NALORE 
=f 
[APRIL 30, 1914 
coloured. Almost immediately a circulation is set up 
and a stream leaves the part warmed by the hand, and 
crossing the film diametrically, curls round on either 
side, producing a tree-like pattern in other colours. 
The film is very sensitive to temperature changes. 
ie ey Oras 
66 Victoria Street, London, S.W., April 24. 
X-Ray Spectra. 
J. Herwec (Berichte der Deutschen Physikalischen 
Gesellschaft, Heft 1, 1914) using a crystal of gypsum, 
obtains by means of the photographic method the 
result that the a and £ lines of the tungsten X-ray 
spectrum coincide with the a and f lines of the 
platinum X-ray spectrum, the values of the glancing 
angles being 4° 56’ and 4° 16’ for the a and # lines 
respectively. But there is reason to suppose that the 
wave-length of X-rays characteristic of an element 
varies inversely as the square of the atomic weight 
of that element. If this is so, we can calculate from 
the experimental results for a platinum antikathode 
obtained by Herweg the glancing angles for the 
tungsten X-ray spectrum and we obtain :— 
Tungsten Antikathode (Calculated). 
Spectrum line a B y 8 € 
Glaticing angle 5° 34’ \4° ‘48! “4° 4904S sG as ene 
We thus see that the calculated values of the B and 
6 lines are nearly the same as the experimental values 
found by Herweg for what he calls the a and # lines 
of the tungsten spectrum. G. E. M. JAauncey.~ 
Physical Laboratory, University of Toronto, 
March 30. 
An Optical Illusion. 
I HAveE often noticed the phenomenon mentioned by 
Mr. J. W. Giltay in Nature of April 23 (p. 189). In 
the position in which he was reading the sunlight 
passed through his eyelids and the coats of his eyes, 
and on account of having to pass through a layer of 
blood which acted as a red screen his retinas become 
flooded with red light. The red is not noticed where 
it is diluted with white, but the print appears red 
because red light is falling on the portions of the 
retina which receive the images of the printed letters. 
F. W. EpRIDGE-GREEN. 
SOME LIFE-HISTORIES AND HABITS OF 
INSHETS4 
(1) T N his “Insect Biographies,” Mr. J. J. Ward 
has written a pleasant and popular account 
of the life-history of several of our best-known 
insects. He has contrived to show how full of 
interest are the facts concerning the growth, 
development, and general mode of life of the 
subjects of his pen and camera, without over- 
burdening his pages with technicalities, or, on the 
other hand, being guilty of inaccuracy or loose- 
ness of statement. The photographs with which 
the book is plentifully illustrated have been in 
almost every instance taken directly from living 
specimens in their natural attitudes and surround- 
ings, and their execution must have involved the 
1 (1) ‘Insect Biographies with Pen and Camera.” By John J. Ward. 
Pp. 206+plates. (London: Jarrold and Sons. 1013.) Price 6s. net. 
(2) ‘‘ Lebensgewohnheiten und Instinkte der Insekten bis zam Erwachen 
der sozialen Instinkte.’’ Geschildert von O. M. Reuter. Vom Verfasser 
revidierte Uebersetzung nach dem schwedischen Manuskript besorgt von 
A.u. M. Buch. Pp. xvi+448. (Berlin: R. Friedlander und Sohn, 1913.) 
Price 16 marks. 
