632 
NATURE 
[FEBRUARY 5, Ig14 
plosive eruption such as excavated these gigantic 
craters, its first effect will be to fill up the crater 
before overflowing the edges. 
Lateral outpourings can only occur when the cone 
has been sufficiently rebuilt, above the level of the 
surrounding country, to give enough hydrostatic force 
to rend this cone. 
The radiating rays around these craters cannot be 
lava streams, as these only flow out of the crater by 
its lowest lip. They are not due to landslips of the 
loose ejecta collected on the slopes of the cone, such 
as I described and figured in my book on the great 
Vesuvian eruption of 1906, and which had until then 
been attributed to water erosion, for the following 
reason. These ravines, like the depressions around 
a half-opened umbrella, are straight radially and not 
sinuously radial as in those surrounding the great 
craters of the moon. 
Were these radial rays lava streams, which origin- 
ally issued from a cone now truncated by a later 
explosive eruption, then they would have been 
obliterated by the enormous mantle of fragmentary 
materials that would have been ejected. 
These rays have more the appearance of erosion 
valleys, but this we cannot admit if physicists main- 
tain that there is no lunar atmosphere to speak of. 
Their greatest resemblance, however, is with the 
irregular, radial cracks formed around the splash of 
a missile striking a comparatively hard surface, such 
as is observable when bullets are fired into soap, hard 
clay, lead, or half-set plaster, or even steel. 
The more I compare the moon’s surface with vol- 
canic vents in different parts of this world the less I 
see a resemblance between the two, and the more 
does the planetoid and meteorite projectile theory 
become acceptable. The obviously asymmetrical 
craters with high, overhanging, narrow lip on one 
side, and low, broad lip on the opposite side, point 
to the impact of the meteorite being oblique to the 
moon’s surface. The long, deep furrows, such as 
the valley of the Alps, &c., are, to my.mind, formed 
by bolides ploughing in a path of high ellipticity the 
surface of the moon, but at so low an angle as not to 
penetrate its surface. 
I think it a great pity that a good lunar-observing 
astronomer with one of the most powerful telescopes 
at his disposal, does not collaborate with a thoroughly 
practical vulcanologist to examine many of the lunar 
features without very rigidly fixed preconceived ideas. 
How often have I wished to be able to study care- 
fully the moon’s surface, and no doubt astronomers 
have often craved for a more extensive vulcanological 
knowledge. H. J. Jounston-Lavis. 
Villa Lavis, Beaulieu-sur-Mer, January 26. 
The End-product of Thorium.—A Suggestion. 
THE chemical composition of thorites and thorianites 
does not seem to suggest any probable end-product for 
the contained thorium. It has occurred to us that 
the only explanation at present available is that the 
end-product is an isotope of thorium itself. This con- 
dition might be brought about by the emission of 
sufficient B rays. 
If this be the case, thorium, as we know it, must 
be a mixture of two isotopic elements, one of which is 
radio-active. There is some support in favour of this 
suggestion to be found in the erratic position of 
thorium on the Geiger-Nuttall curve (Phil. Mag., 
October, 1912). According to this curve, the value of 
X for thorium, as observed, is too low. Now, if there 
is a stable component present, this result will naturally 
arise. 
From the position of thorium on the diagram it is 
possible to estimate the value of A for the active con- 
NO. 2310, VOL. 92] 
stituent on the above hypothesis. It comes out 
approximately as 1-0x 10~-**sec-*. The percentage of 
this active constituent would appear to be about 0-7. 
It is also possible to estimate the time for this com- 
position to have been attainedystarting from the pure 
active constituent. The time appears to be about 
1-6 x Io" years. \ 
The view that thorium possesses a radio-active con- 
stituent as determined above may, of course, be made 
the basis of an independent hypothesis. d 
Jeahonss i 
J. R. Correr. 
Trinity College, Dublin, February 3. 
A Curious Ice Formation. 
I am taking the liberty of enclosing a photograph 
of an occurrence which, so far as | am aware, is 
quite unique for this part of the country, and will no 
doubt have some interest for your readers. 
The water was frozen during the night of December 
31, 1913 (on which night at least 14° of frost were 
registered) into circular floes of ice of varying 
diameter, which, being encrusted with snow, had the 
appearance of water-lilies. 
Photo.) { J. Clark, Brecon. 
The river at this point flows almost due southward, 
and has just passed under a bridge over a weir, at 
both ends of which is a whirlpool. 
The accompanying photograph shows the east 
whirlpool as it appeared on New Year’s Day. 
The river, I may mention, is the Usk, and the 
photograph was taken at Brecon. : 
‘Di De Ea 
University College of South Wales and 
Monmouthshire, Cardiff. 
Soil Protozoa. 
In a letter to NaTuRE (No. 2266, vol. xci., 1913) one 
of us (C. H. M.) gave an account of a method of 
obtaining permanent preparations of Protozoa in the 
state in which they were living in the soil. 
The fixative used in this method was picric acid in 
saturated aqueous solution, but we have since found 
this reagent to be less serviceable in the case of clay 
soils than the following mixture :—Saturated aqueous 
solution of mercuric chloride, 1 pt.; methylated spirit, 
1 pt. The soil should be crumbled into this fluid, and 
mixing is best accomplished by gently shaking the 
containing vessel, care being taken to avoid making 
the clay component of the soil pass into suspension. 
A delicate film containing Protozoa will appear on 
the surface of the liquid, and this can be removed by 
floating cover-slips over it, and stained by the usual 
methods, K. R. Lewin. 
C. H. Martin. 
Lawes Experimental Laboratory, Rothamsted, 
January 27. 
