May 20, 1886] 
NATURE 
67 
otherwise alter until the field quickly became dark owing to the 
sion of the crystal. This seems to indicate that the change in 
begins quite independently of the formation of the 
Teflective lay~ the latter being only an incident occurring at a 
‘particular stage of tne Saino 
f (6) It would seem that Sim hing of the following kind 
happens to the crystal. Itis, of course, exicotropic in structure, 
and the effect of heat is to set up a molecular strain which at a 
certain point of temperature causes so strong a shearing actian 
between nearly contiguous layers of the substance that whole 
rows of crystal elements lying between these layers are rolled 
over, as it were, by the ‘‘couple” applied to them, until they 
take up their ‘‘second positions of equilibrium,” as M. Mallard 
would say (see his paper ‘Sur la Théorie des Macles,” Au/?. 
Svc. Min., December 1885, p. 467). If these latter positions 
were such as to bring the od¢wse bisectrices (supplementary lines) 
into parallelism with a normal to the main plate, the occurrence 
of the hyperbolas abuve described would be fully accounted for. 
‘Such an action would be of the same general character as that 
which takes place in cale-spar when macles are being developed 
n it by Keusch’s method ; viz. by carefully compressing a crystal 
of it in a definite direction (ogg. Ann., vol. cxxxil. p. 445), 
I have succeeded by properly regulating the direction and 
amount of the pressure in making spar-macles containing 
numerous ‘‘planes of sliding” (Gveitfachen, as Prof. Reusch 
‘calls them), which reflect light with a pearly lustre, and almost 
as brightly as the potassium chlorate macles described above. 
_ It has yet to be explained, however, why the intense reflective 
power does not show itself during the process of heating, when 
the tilting over of the crystals would certainly take place, and not 
juntil a particular stage of the cooling is reached. I am inclined 
‘to believe that this may be due to the substance acquiring a 
‘certain amount of plasticity at high temperatures, such as has 
‘been observed by M. Mallard in crystals of nitre under similar 
‘circumstances. This may prevent any loss of optical continuity 
until a certain critical point in the cooling has been reached ; 
and at this point the displaced crystal elements suddenly part 
‘company with their unaltered neighbours, leaving a numerous 
series of parallel tubular cavities, precisely like those which are 
‘undoubtedly present in calc-spar macles formed by Keusch’s 
method. The opposite sides of these parallelogrammatic cavities 
may be so near each other that the rays reflected from them may 
‘interfere, and give the colours of thin plates corresponding to a 
lrather high order in Newton’s scale. Although a large amount 
‘of light must escape reflection at any single cavity, yet if the 
‘transmitted rays encountered a large number of precisely similar 
jand similarly situated cavities at slightly lower levels in the 
‘crystal, the sum of the partial reflections would produce an effect 
almost equivalent toa total reflection of the original incident ray, 
md a corresponding deficiency in the amount of light trans- 
mitted through the whole plate. ‘The brilliancy of the colours 
in the light reflected from the well-known films of decomposed 
Jass is accounted for in precisely the same way, and the suc- 
ssive separate films of glass can be easily seen under a micro- 
See. 
scope at the edges of the compound film, .where they only | 
partially overlap. 
_ The fact that no brilliant reflection is observed in and near the 
lane of symmetry of the crystal may be due to the sides of the 
avities in a given horizontal row not lying strictly in the same 
Jane, but being slightly inclined alternately in opposite direc- 
ions, so as to form a series of anticlinals and synclinals, or 
idges and furrows like those of a roof. Thus a beam of light 
ncident in the plane of symmetry would be reflected in directions 
lying a little to the right and left of this plane, and not in the 
lane itself. The satin-like appearance of the reflecting layers, 
ready alluded to, would be fully accounted for by such a struc- 
ture. 
_ The changes above described seem of interest as bearing upon 
the cause of the strong iridescence of some crystals of potassium 
2. . . . 
hlorate, about which I may have something to say in a future 
ommunication, H. G. MADAN 
Eton College, May 10 
S SCIENCE IN RUSSIA 
THE last volume of the A/emozrs of the Kharkoff Society of 
ae Naturalists (vol. xviil.) contains several papers of interest, 
‘All who have had to deal with Acarides, and are acquainted 
with the difficulties of their classification, will welcome the 
elaborate memoir, by M. Krendowsky, on the Hydrachnids of 
ty 
Southern Russia. It is not a mere description of forms, with 
a more or less happy classification, but an elaborate contribution 
towards the systematic arrangement of this imperfectly-known 
subdivision. The embryogeny of the Hydrachnids, and especially 
their larval phase, have received special attention, no satisfactory 
classification being possible without that preliminary study. It 
appears also from M. Krendowsky’s researches that many 
Hydrachnids of Southern Russia are really temporary parasites 
on several insects, mollusks, and sponges, especially when 
young and in the state of six-footed larvae. The Hydrachnids 
of South-Western Russia belong to thirty-five. species (nine 
species each of Wesea and Arrenurus, five of Adax, and four of 
Limnesia) ; the author has been led to revise the whole of the 
classification of the freshwater Acarides, and gives it complete, 
with analyses of each family, as well as of the very numerous 
genera. 
Another paper of great interest is devoted by the same author 
to the estuaries of the Bug, Dnieper, and the smaller ones in the 
neighbourhoods of Kherson and Odessa. ‘This paper is full of 
the most useful information as to the characters and geological 
history both of these estuaries and the Zmans, which are now 
shut off from the sea by their sand-bars, and have become mere 
elongated salt lakes. 
Prof. Lewakowsky contributes to the same volume a paper on 
the Jurassic limestones of the Crimea, based especially on their 
micro-structure, It appears that they mostly contain very small 
debris of corels and rhizopods ; they are not coral structures, as 
was supposed, but have much likeness to what Dana describes 
as beachsand-rock. Like the clay-slates of the same formation 
in the Crimea, they have been deposited in a wide basin which 
extended into Kherson and Ekaterinoslay, and they were com- 
posed of materials brought from the south, from a continent 
which occupied part of what is now submerged by the Black 
Sea. M, Genjouriste’s researches into the microscopical struc- 
ture of the coal of the Donez Basin are interesting inasmuch as 
they show that the prevailing materials for the formation of this 
coal were the higher vascular Cryptogams, and not Algze, as 
was sometimes supposed by Kussian geologists. Dr. M. 
Dybowsky’s additional note on the Spongilla Dorvilia slepanovit, 
one of the most interesting discovered in Europe, contains a 
description of the structure of its gemmulz, with the porous and 
“*cirrous appendages.’ The note, as also the preceding papers, 
are accompanied by several plates. 
SCIENTIFIC SERIALS 
The Quarterly Fournail of Microscopical Science, vol. xxvi. 
part 3, April 1886, contains a memoir on the leeches of Japan, 
by Dr. C. O. Whitman (plates 17 to 21). A short abstract of 
this important memoir has been given in our Biological Notes. 
—Contributions to the embryology of the Nemertea, by Prof. 
A. A. W. Hubrecht (plate 22). No. 1 is an account of the 
development of Linens obscurus, Barr. These investigations, 
already published in the Dutch language, are fully detailed in this 
paper, and the plate gives the details of the principal results, 
combined into fifteen diagrammatic tracings. In one section 
the earliest developmental stages and the derivatives of the primary 
epiblast ; in a second the hypoblast before the shedding of the 
primary larval integuments; and in a third the mesoblast, are 
treated of.—On the early development of /zlus terrestris, by 
F. G. Heathcote, M.A. (plates 23 and 24). This is the first 
part of an essay on a subject not treated of by British naturalists 
since the days of Newport. It treats of the segmentation of the 
ovum, which shows a remarkable resemblance to that found in 
Amphipods by Uljanin. The formation of the blastoderm is 
such as is generally found in tracheate development. ‘The cells, 
which at the conclusion of the blastoderm formation remain 
within the yolk, represent the endoderm, The mode of forma- 
tion of the mesoderm almost exactly resembles that described by 
Balfour for spiders. In a future paper the author intends de- 
scribing the further developmental stages of the embryo.— 
William A. Haswell, M.A., on the structure of the so called 
glandular ventricle (Driisenmagen) of Syllis (plate 25). This organ 
is in reality a well-developed muscular gizzard, and contains no 
glands in its walls. The muscular elements of the organ pre- 
sent an embryonic character containing as they do a polynucleated 
core.—Arthur B. Lee, on Carnoy’s cell researches (plate 26). 
While Carnoy’s conceptions of the cell body do not materially 
differ from received views, the author of this paper thinks that 
sufficient attention has not been given to his labours on the 
