ne ee ae 
June 9, 1923] 
NATURE 
773 

the matter here. What may be called second order 
scattering does not work very well on the hypothesis, 
but neither does it on the classical theory. Rather 
ersely the phenomenon which causes almost 
insuperable difficulty is the one which is most satis- 
factory on either the classical or the quantum theory, 
and that is the phenomenon of resonance radiation, 
as exhibited in Wood’s work with mercury vapour. 
On my hypothesis the vapour ought to be excitable 
by light of wave-lengths different from its own, 
instead of requiring a very exact adjustment in the 
incident light, as it in fact does. It seems possible 
that a satisfactory modification of the hypothesis 
pont result from a study of this failure. 
n connexion with resonance radiation it is worth 
raising the question of whether the resonant light. has 
a phase relation with the incident. In the quantum 
theory it is always assumed that it does not, but 
there does not seem to be much direct evidence. 
As pointed out above, there must be some light 
scattered in the process of absorption, and this light 
must have a phase relation, but it would depend on 
the phase difference whether this is the observed light 
or only a much weaker emission of the same fre- 
quency. I suppose the balance of evidence is rather 
against the phase relation; on that side there is the 
fact that one line of the spectrum can excite the 
emission of others, and there is some indication of the 
existence of a considerable latent period. On the 
other side any form of the wave theory requires that 
at least a part of the scattered wave should be in 
phase, and there is also some support, though not 
very strong, in Wood’s recent discovery that the light 
is nearly completely polarised. Perhaps the work of 
Clark and Duane may also be invoked on this side. 
As general conclusion the argument shows that the 
physical picture associated with the present quantum 
theory can be valid only over a very limited field, and 
that the more satisfactory parts of the electromagnetic 
theory can be taken over by a wave theory freed 
from many hampering restrictions. 
C. G. DARWIN. 
Institute of California, 
Pasadena. 
The so-called ** Baccy-juice’’ in the Waters 
of the Thames Oyster-beds. 
DurinG May or June the waters over the oyster- 
beds at various places in the Thames estuary become 
riodically brown-coloured. This brown coloration 
is called “‘ baccy-juice ’’ by the local fishermen, who 
have connected with it such important observations 
on fisheries that its nature is worth recording. By 
the courtesy of Major A. Gardner and Mr. Louis 
French, I obtained on May 24 and May 28 tow- 
nettings and living samples of the “ baccy-juice”’ 
from off Whitstable and off West Mersea, and find, 
as surmised, that the brown coloration is due almost 
entirely to the presence of great numbers of the 
spherical colonies of the brown flagellate Phocystis. 
It is well known that Phzocystis occurs periodically 
in the English Channel and in the North Sea, and it 
is not surprising that it should occur in a similar way 
in the Thames estuary. The occurrence of “ baccy- 
juice’ in the Thames estuary is not welcomed by 
the fishermen (excluding oyster fishermen), who say 
that it is useless trawling for fish in the locality of 
this material, and also state that a cold spell of a few 
days is sufficient to cause it to disappear; these 
apparently good practical observations are well worth 
recording. 
The Pheocystis from both sides of the Thames 
estuary, it is interesting to note, were carrying on 
NO. 2797, VOL. 111] 
each colony two or three individuals of a species of 
Acineta, closely allied to if not identical with Acineta 
tuberosa, var. fraiponti, which is taking advantage of 
the floating Phzocystis to adopt a planktonic and 
semi-parasitic habit. 
A brown coloration of the water over oyster-beds 
in the riverine portion of estuaries is also very 
common in summer and autumn, but in the rivers 
Yealm and Helford and in the Hamoaze estuary this 
colour is due almost entirely to various peridinians, 
which constitute a very large proportion of the diet 
of oysters at this time. In an estuary more open to 
the influence of the sea, where high salinities probably 
occur, as at Padstow, a brown coloration in autumn 
was found to be due to enormous quantities of a 
species of Chetoceras. In July 1922 the slight brown 
coloration of the water over the oyster-beds in the 
West Mersea creeks of the River Blackwater was due 
to a variety of diatoms, among which Leptocylindrus 
danicus,, Cleve, was the most common; but at the 
same time the diatom, Nitzschia closterium, was the 
dominant and almost the only floating form in those 
stagnant or semi-stagnant oyster-pits which had mud 
on the bottom. J. H. Orton. 
Marine Biological Laboratory, 
Plymouth, May 28. 

The Relation of the Critical Constants and the 
True Specific Heat of Ferromagnetic Substances. 
A MAGNET should have, like a fluid, three critical 
constants—a critical temperature, a critical intensity 
of magnetisation, and a critical field. The critical 
temperature and the critical intensity may be ex- 
perimentally determined ; the critical field is more 
difficult to find by experiment, but it may be calculated 
from the other two critical constants. When this 
is done for iron, cobalt, nickel, and magnetite it is 
found that the critical fields are very simply related 
to one another, being almost exactly in proportion 
to the numbers fo (1°5), 2°0, and 3:0 respectively. 
Further, these numbers are inversely as the true 
specific heats of these substances at their critical 
temperatures, and the product of the critical field 
and the true specific heat must therefore be a constant. 
For iron this constant is 0°0225, for cobalt 00230, 
and for nickel 0'0225 ; for magnetite it is o-o69t, but 
if this is divided by 3, the number of atoms of iron 
in the molecule, the result again is the number 0°0230. 
The critical field is calculated as #/8I,, where @ is 
the absolute critical temperature, and I, the maximum 
intensity of magnetisation; and hence the true 
specific heat multiplied by the ratio @/I, is 00225 x 8, 
that is, 0°18. Now this number is, to a close ap- 
proximation, five times the energy per unit of 
temperature for one degree of freedom calculated 
from R, the gas constant, and the atomic weights of 
the ferromagnetic metals, and this points to the 
specific heat at the critical temperature as that 
corresponding to five degrees of freedom. As there 
are three degrees of freedom in the calculation of 
the specific heat at air temperature, there must be 
an acquisition of two degrees of freedom at the 
critical temperature, a conclusion which has been 
reached by a different method, and was the subject 
of a letter printed in NAruRE of July 1, 1922 (vol. 
II0, p. 10). 
The result stated above may be put in another 
way by saying that the thermal energy at the critical 
temperature and the maximum intensity of magnetisa- 
tion of the ferromagnetic substances are proportional 
to one another. 
1 I am indebted to Dr. M. V. Lebour for this identification, | 
