428 
Large-scale Map of the Salonika Battle Front. 
(London: J. W. Bacon and Co., Ltd.) Price 
Is. net on paper, 1s, 6d, net on cloth. 
THis map, on a scale of 5 miles to 1 in., shows 
the country to the north of Salonika as far as 
lat. 42° 10’ N., and east to Kavalla and west to 
Monastir. It is layer coloured, and the contour 
interval is tooo ft. International boundaries 
are clearly shown, and a red line indicates the 
approximate position of the Allies’ front. Rail- 
ways and main’ roads are shown, as well as 
swamps. The map is carefully executed and con- 
tains plenty of names. No doubt it could be 
improved by the addition of a 500-ft. contour 
line, but it is the best cheap war map of this. 
region which has yet appeared. 
LETTERS. TO THE EDITOR. 
[The Editor does not hold himself responsible for 
opinions expressed by his correspondents. Neither 
can he undertake to return, or to correspond with 
the writers of, rejected manuscripts intended for 
this or any other part of NaTuRE. No notice is 
taken of anonymous communications.] 
Talbot’s Observations on Fused Nitre. 
AmonG the litthke remembered writings of that re- 
markable man, H. F. Talbot, there is an optical note 
in which he describes the behaviour of fused nitre 
(nitrate of potash) as observed under the polarising 
microscope. The experiments are interesting and 
easily repeated by anyone who has access to a suit- 
able instrument, by preference one in which the 
Nicols can be made to revolve together’ so as to 
maintain a dark field in the absence of any interposed 
crystal. 
“Put a drop of a’ solution of nitre on a small 
plate of glass, and evaporate it to dryness over a 
spirit-lamp; then invert the glass, and hold it with 
the salt downwards and in contact with the flame. 
By this means the nitre may be brought into a state 
of fusion, and it will spread itself in a thin trans- 
parent film over the surface of the glass. 
“Removed from the lamp it immediately solidifies, 
and the film in cooling cracks irregularly. As soon 
as the glass is cool enough, let it be placed beneath 
the microscope (the polarisers being crossed, and the 
field of view consequently dark).” 
I have found it better to use several drops spread 
over a part of the glass. And instead of inverting 
the plate in order to melt the nitre, I prefer to employ 
the flame from a mouth blow-pipe, caused to play 
upon the already heated salt. ‘The blow-pipe may 
also. be used to clean the glass in the first instance, 
after a preliminary heating over the flame to diminish 
the risk of fracture. Further security is afforded by 
keeping down the width of the strip, for which half 
an inch suffices. 
Talbot describes how, under the microscope, there 
appear crystalline plates of irregular shape, often 
fitted together like a tesselated pavement, each plate 
forming a single crystal. If one plate is darkened 
by rotation of the Nicols, the others remain visible in 
varying degrees of brightness. If the plates are thin, 
the light is white; but with more salt they display 
_ colour, and the colour is not always uniform over 
the whole plate, indicating a variable thickness. But 
this condition of things is not permanent. After per- 
haps a quarter of an hour the plates break up ina 
surprising fashion, and the general appearance is 
totally changed. i ‘ 
NO. 2466, vor. 98] 
NATURE 
[FEBRUARY I, 1917. 
Moreover, the transformation may be accelerated. 
“Let a film of fused nitre be obtained in the manner 
already mentioned, and let it be allowed to cool during 
three or four minutes. The plate of glass should be 
turned round upon the stage of the microscope until 
the crystalline film is darkened as accurately as pos- 
sible. Things being thus adjusted, Jet the observer 
touch the film with the point of a needle while he 
is observing it in the microscope. He will perceive 
that the touch immediately produces a luminous spot 
on the dark surface, and this spot will slowly expand 
itself in all directions like a luminous wave. This 
is a very curious object, but difficult to describe.” 
And further on :—‘‘If, however, we touch it pre- 
maturely, as, for instance, during the first minute 
after it has become solid, this change does not take 
place.” 
I have made a few trials to ascertain whether the — 
life of the plates can be prolonged. Protection from 
atmospheric moisture did little good. Another plate, 
kept for five hours at a temperature not much short 
of that of boiling water, was found to have undergone 
transformation. But, as might be expected, a higher 
temperature over a diminutive gas flame acted as a 
safeguard, and the plate after removal behaved like 
one newly formed. 
I have found that nitre may be replaced by chlorate 
of potash, with the advantage that the plates will 
keep (at any rate in an artificially warmed room) 
for weeks, and perhaps indefinitely. The appearances 
are similar, but less beautiful, as colour is not so 
often developed. The chlorate is more fusible than 
nitre, and the heat should not be pushed beyond what 
is needed for fusion. : 
Other salts—for example, silver nitrate—which fuse 
in the anhydrous state without decomposition may 
also be employed, as is probably known to those who 
prepare objects for the microscope. But Talbot’s early 
observations on nitre are rather special, and deserve 
recall as they seem to be but little known. 
RayLEIGH. 
** Plants in Health and Disease.’’ 
On p. 331 of Nature of December 28, 1916, the 
writer of the review of the book entitled ‘‘ Plants in 
Health and Disease’ remarks :—‘‘The accounts of 
such pests as the cabbage-root fly and the onion fly, 
which have been very active this year, are particularly 
clear. We could only wish that the measures whereby 
these pests are to be combated were half as good.” 
With reference to the cabbage-root fly, I am glad now 
to be able to report that an efficient measure for deal- 
ing with that widespread pest has been tested under 
my direction during the past season. 
Mr. J. T. Wadsworth, research assistant in this 
department, has conducted a series of experiments 
with American tarred felt paper discs, and a_ full 
account of the work will appear in the next issue of 
the Annals of Applied Biology. The tarred felt discs 
each measure 24 in, square, and are provided with a 
slit which enables them to be slipped round the stems 
of young cabbages and cauliflowers so soon as the 
latter are planted out in the field. Each disc is pressed 
flat upon the surface of the soil round the plant, and _ 
no further attention as a rule is needed. Its primary 
function is to act as a mechanical obstacle, preventing 
the flies from laving their eggs on the soil in the 
usual position, close around the plant. Out of 816 
cabbages used in these experiments, half of them were 
provided with the discs, and the remainder left un- 
protected. Only one plant was lost out of the 408 
protected examples, while fifty-four unprotected cab- 
bages were severely attacked. With regard to cauli- 
flowers, the results were even more striking. Alto- 
Ch a 
_—— 
