5 
on 
8 NATURE 
[APRIL 11, 1907 
A section is devoted to the insects which prey upon 
plants, and to the measures to be talken for the de- 
struction of these pests, as well as of fungi. That 
the book is up to date may be gathered by the refer- 
ences to Mendelism and De Vries. 
A copious index is given, as well as hints as to 
the way in which examination questions should be 
answered. 
A little more information as to the “reason why ”’ 
of digging, watering, striking cuttings, and other 
garden operations would have increased the value of 
the book, which nevertheless is one which can con- 
fidently be recommended to the attention of all those 
interested in gardening. 
Dr. Schlich’s Manual of Forestry. Vol. iv. _ Forest 
Production. By W. R. Fisher. Being an English 
adaptation of “* Der Forstschutz,” by Dr. Richard 
Hess. Second edition. Pp. xxiii+712. (London: 
Bradbury, Agnew and Co., Ltd.) 
Tuts volume is the second edition of Prof. Fisher’s 
** Forest Protection,» and is uniform with the third 
edition of vols. i., ii., and iii. of Dr. Schlich’s 
“Manual of Forestry.”” The book is an English 
adaptation of Dr. Hess’s ‘ Forstschutz,”” that is, it 
is not a mere translation, as the author has exercised 
discretion in his selection of material in order to 
make the book more adapted to the use of British 
and Indian foresters. New illustrations have also 
been added which are not in the German edition. 
The subject of forest protection is of immense import- 
ance, and covers a wide field of knowledge, prac- 
tically including every branch of scientific sylvi- 
culture. The author has arranged and presented 
the various protective measures to be adopted against 
inimical agencies, both in the organic and inorganic 
worlds, in a very clear and interesting manner. The 
volume also contains a useful index at the end. Prof. 
Fisher has done valuable work by rendering avail- 
able to student and forester a vast store of inform- 
ation which has hitherto been accessible only to a 
few. The book is one which we can warmly recom- 
mend to all those who have forests or trees under 
their charge. 
The Essentials of Histology, Descriptive and Prac- 
tical. By Prof. E. A. Schafer, F.R.S. Seventh 
edition. Pp. xi+s507. (London: Longmans, Green 
and Co., 1907.) Price ros. 6d. net. 
Tue fact that this volume has reached its seventh 
edition shows conclusively that it supplies a want. 
The features of the present edition are the introduc- 
tion of colouring in the illustrations and a consider- 
able increase in the part devoted to the nervous 
system. In this portion practically a new set of 
illustrations appears, which can only be described as 
admirably calculated to indicate the salient points 
which the elementary student must be familiar with. 
Either for the purely scientific or for the medical 
student this book will continue to be of the highest 
value. 
Actualités scientifiques. By Max de Nansouty. 
Pp. 361. (Paris: Schleicher Fréres, 1906.) Price 
3-50 francs. 
THE general character of this annual publication was 
described in noticing the issue for 1905 in NaTURE 
of November 23, 1905 (vol. Ixxiii., p. 76). The short 
essays on scientific subjects of current interest range 
over most branches of science, and should be useful 
as reading exercises in French classes in schools 
where the pupils also learn something of science. 
NO. 1954, VOL. 75] 
LETEERS LO) 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.] 
A Hydraulic Analogy of Radiating Bodies for Illus- 
trating the Luminosity of the Welsbach Mantle. 
Tue device about to be described enables us to illustrate 
to a class the behaviour of different types of radiating 
bodies when introduced into a flame, and will be found 
especially useful in explaining the remarkable luminosity 
of the incandescent mantles used in modern gas-lighting. 
It is, of course, not intended to explain the mechanics 
of radiation, but merely to enable us to describe certain 
phenomena in terms of easily grasped notions. 
Students are told that the more powerfully a body 
absorbs the more powerfully will it emit when heated, 
this relation holding for every individual wave-length. 
Black bodies, then, give out the most light when heated. 
The fact that a white block of lime is far more luminous 
than a carbon rod when heated in the oxyhydrogen flame 
is not usually cited in support of this law, while the fact 
that the most luminous body of all, the Welsbach mantle, 
is also quite white, is equally unsatisfactory as an illus- 
tration, for white bodies are in reality transparent, that 
is, they are made up of masses of small transparent par- 
ticles, and transparent bodies ought not to emit at all. 
It is, of course, necessary to define just what we mean 
by transparency in this case, and it may be well to con- 
sider first a somewhat analogous case. The absorption 
which is accompanied by high emissivity is true absorp- 
tion, and not selective reflection, which is sometimes con- 
fused with absorption. A highly reflecting polished metal 
surface is a poor radiator, but by properly constructing 
its surface we may give it the power to absorb and emit. 
A bundle of polished steel needles with their points all 
turned towards the source of light reflects scarcely any 
light at all, the rays undergoing multiple reflections 
between the conical ends of the needles. Such a bundle 
of needles should emit much more powerfully than a 
polished steel surface, and it is easy to see just why it 
should do so. Each needle, seen end on, sends not only 
emitted light to the eye, but reflects rays coming from its 
neighbours. The surface formed by the points of the 
needles can be regarded as an absorbing surface, which 
absorbs in virtue of its structure; it is analogous to the 
hollow ‘‘ black bodies ’’? with which we are now familiar. 
The point which I wish to emphasise is that such a 
surface, which absorbs not at all in virtue of its mole- 
cular nature, is also a powerful radiator, the mechanism 
by which its radiating power has been increased being as 
indicated above. 
Suppose, now, we take a perfectly transparent body, 
which, like a perfect reflector, has no emitting power. A 
bead of microcosmic salt (sodium pyro-phosphate) heated 
in a blast lamp is a good example. Though the platinum 
wire which supports it glows with vivid incandescence, 
the bead remains perfectly dark. A glass bead, however, 
emits a good deal of light, doubtless from the fact that 
its transparency is much less at high temperatures, a very 
common behaviour of transparent substances. The micro- 
cosmic salt on cooling becomes traversed by hundreds of 
cleavage planes, which give it a milky appearance. On 
re-heating it it emits light strongly, until it finally fuses 
into a transparent drop, when it instantly becomes dark 
again. The reason for this behaviour is not quite so 
apparent as in the case of the needles. In fact, I am 
not quite sure that I understand it at all. Quartz behaves 
in the same way. A drop of clear fused quartz, heated in 
the blast, emits little or no light, but if it contains spots 
made up of an emulsion of quartz and air, these spots 
emit strongly. In other words, an opacity resulting from 
a pulverisation of the transparent medium seems to be 
accompanied with a strong emitting power. Apparently we 
cannot apply the same reasoning as in the case of the 
needles, and it looks rather as if the radiation was largely 
a surface effect. If this is so, it is obvious that an 
