246 
NATURE 
[JuLy 10, 1902 
country, it may be worth while noting that it does not 
show the total humus, but only the humic acids. 
Berthelot has, in fact, shown that even boiling with 
potash leaves a considerable part of the organic carbon 
and nitrogen of a soil undissolved. 
English agricultural writers employ two names for 
Beta vulgaris—“ mangel” and “mangold”; Mr. Ingle 
employs the latter. The former spelling is, however, 
more correct. The word comes from the German de- 
scription “ Mangel-Wurzel,” or scarcity root, alluding to 
its resistance to drought. The spelling has probably 
drifted into mangold from the golden colour common _ to 
the roots. R. W. 
By F. Hilde- 
W. Engelmann, 
Ueber Aehnlichketten im Pflanzenreich. 
brand. Pp. iv+66. (Leipzig: 
1902.) Price Is. 9d. net. 
PROF. HILDEBRAND, in his introductory remarks, takes 
exception to the use of the term “mimicry.” He states 
that it is applied by zoologists when two very different 
animals show similar appearances which are of apparent 
benefit to one, and that the explanation of zoologists 
infers that these similarities are developed in the struggle 
for existence. The latter part of this statement is dis- 
tinctly misleading, as it is doubtful if any zoologists 
regard such similarities as being developmental. The ob- 
ject of the book is to show that in the plant world mimicry 
rarely if ever occurs, and that similarities in plants or 
plant forms are mainly due to environment or ecological 
factors. The series of comparative sketches which Prof. 
Hildebrand has published form light reading, but they 
might with advantage have been worked up in greater 
detail. 
Index to the Literature of the Spectroscope (1887-1900, 
both inclusive). By Alfred Tuckermann. Pp. 373. 
Continuation of the previous index by the same author 
published in 1888. (Published by the Smithsonian 
Institution, 1902.) 
IN the previous index, extending from the dawn of 
spectroscopy (or even earlier, for references are made to 
papers published in the seventeenth century) to 1886, the 
author arranged the books and papers under 320 different 
sections, placed alphabetically. In each section the titles 
of the papers, the authors’ names, and references to the 
original papers and abstracts are arranged in the alpha- 
betical order of the authors’ names. The present contri- 
bution is divided into two parts, part i. being an authors’ 
index extending to 188 pages, in which the authors’ 
names are placed alphabetically and the full title, year 
of publication and references to the papers and abstracts 
are given; and part il. a subject-index beginning with 
history, books, spectroscopy in general, followed by 
nearly 300 divisions arranged alphabetically. In these 
divisions the authors’ names are first given alphabeti- 
cally, followed by the references to the papers with the 
year of publication, but without any reference to the 
titles or to contents of the papers which are not given in 
the titles. Thus under titanium there are five references ; 
the first is in the Wrenexr Anzezger, and does not appear 
in the author-index, the second is on ultra-violet spark 
spectra, the third on titanium as a comparison spectrum, 
the fourth on the arc spectrum, and the fifth on the shifting 
of the arc spectrum lines under the influence of pressure. 
The value of the index would have been enormously 
increased if the papers had been arranged alphabetic- 
ally according to the subjects, and with the papers 
on the same subject placed in order of date instead of 
according to the authors’ names. Such a system would 
have entailed more printing, but it would not have caused 
very much more work in preparation and would certainly 
have been worth the additional trouble. 
The list appears to be very complete ; it may be said 
to be more than complete, for some of the papers in- 
NO. 1705, VOL. 66] 
dexed do not deal with spectroscopy. Thus five papers on 
meteors which we have examined do not contain any refer- 
ence to spectra, and one on the yellow variety of arsenic 
does not deal with spectroscopy; several papers are 
indexed which contain only micrometric measurements 
of the diameters of planets. It is perhaps ungracious to 
criticise in this manner a work which must have been 
very arduous to the author, but the inclusion of papers 
that do not refer to the use of the spectroscope may 
be the cause of much loss of time and trouble to 
workers, and this would not have happened if the subject- 
index had been prepared in the way above suggested. 
Dr. Tuckermann must be congratulated on the con- 
clusion of his work, which, notwithstanding the defects 
which we have mentioned, cannot fail to be of service 
to many investigators in this important branch of 
science. H. M. 
LETTERS ‘OW RAE IE DIMOR: 
(Zhe Editor does not hold himself responsible for opinions ex- 
pressed 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 ts taken of anonymous communications.) 
Misuse of Coal. 
NAtuRE of March 20, containing a most interesting com- 
munication by Prof. John Perry on the ‘‘ Misuse of Coal,” has 
reached me lately. Surely Prof. Perry takes an insular view of 
the matter. Like so many Englishmen, alas! he knows not 
the forest! The greater portion of the world cooks its food 
and makes itself comfortable on wood fuel, and though all the 
forests in the world would, according to European ideas, be 
inadequate to supply by their growth the present expenditure of 
coal (their fossilised remains), to overlook altogether the sun 
power which we can fix by growing wood fuel is surely, from 
even a European point of view, an oversight. Helmholtz com- 
pared the number of thermal units received by an acre of land 
in Germany during a year with the number of thermal units 
produced by burning the vegetable matter elaborated during a 
year. His calculation was that only the 1/1477th part of the 
sun’s heat was thus rendered available. 
On this basis it is possible roughly to calculate the maximum 
thermal efficiency as firewood of the wattle or Eucalyptus 
vegetation on the coast of Australia or South Africa. (Inso- 
lation is for the latitude somewhere about one-sixth greater at 
Cape Town than in mid-Germany ; practically it is more on 
account of the clearer atmosphere.) The production of fire- 
wood is about five times as much ; thus, taking Crottondorf as an 
example of a’ European forest giving one of the largest yields in 
timber, we have :— 
Crottondorf spruce, mean yearly yield 143 cubic feet. 
Quick-growing Eucalypts, S. Africa, do. 700 ,, 
Or the maximum South African yield is five times the 
maximum European yield. But since the average weight of 
eucalypt wood is three times that of spruce, the heating power 
produced on an acre of Eucalypts must be set at about fifteen 
times that produced on an acre of northern and mid-European 
forest. Thus on the basis of Helmholtz’s calculation a eucalypt 
plantation can, with the most favourable circumstances, in 
South Africa or on tropical mountains, store up, say, 15/1500 
= I per cent. of the solar energy received on the unit of area. 
The position in Cape Town to-day is that it is cheaper to 
plough the ground and plant a forest of quick-growing trees than 
to import coal from over the sea or by along and expensive land 
journey. Firewood in Cape Town is worth nearly Is. per 
cubic foot, and before the railway was extended to the diamond 
fields firewood there has fetched 1d. per lb., the price at which 
sugar has been retailed in England. No doubt from a British 
insular point of view coal at 2/. or 3/. per ton is a terrible 
misfortune. It certainly increases the cost of running 
machinery ; but if this does not take place to a prohibitive 
extent, and if it makes the user of power careful not to waste 
it, it is not an unmixed evil. And if thereby afforesting is 
made a paying operation, it is at least open to discussion 
whether dear coal and good forests would not be better for 
England than an expenditure of 23,000,000/. sterling on 
