(i 
Sept. 4, 1873] 
NATURE 
363 
__ tory sketch of the present state of our knowledge of terrestrial 
vulcanicity,” &c., is far from being such, and in greater part but 
a one-sided exposition of Mr. Mallet’s own views of what he 
terms vulcanicity and vulcanology, and which, to quote one of 
his reviewers, “‘has really no connection with Palmieri’s report.” 
It is unnecessary here to occupy space in splitting hairs over 
the exact definition of words, such as theory, hypothesis, force, 
&c., being quite content to assume that the readers of NATURE 
fully understood the sense in which I employed them ; but when 
the abstracts published in the ‘‘ Proceedings of the Royal So- 
ciety” are generally admitted to be faithful reports of the main 
features of the Memoirs read before the Society, and here it is 
not a question of details, I ask any rational individual whether 
Mr. Mallet, merely because he considers the abstract of his 
paper as ‘‘most meagre and incomplete,” is justified in using 
such words (p. 382) as ‘‘Mr. Forbes commences with an im- 
portant error as toa matter of fact, by referring to ‘ Mr. Mallet’s 
Dynamical Theory of Volcanic Energy,’ as published in the 
Proceedings of the Royal Soeiéty for 1872.” 
When an author commits himself to print, he should also be 
prepared for the consequences ; yet the tone of Mr. Mallet’s 
critique evidently indicates extreme irritation in finding his views 
commented upon before his communication to the Royal Society 
is published in full, explaining in other words, that before this 
is done nothing is known about them—a state of things emi- 
nently suggestive, both that the scientific men who, he is pleased 
to inform us, have already expressed themselves in his favour, 
may, after all, have been somewhat hasty in so doing, and also 
that Mr. Mallet would have been more wise if he had withheld 
his self-laudatory sketch until the publication of the evidence in 
favour of his views had afforded the scientific world the opportu- 
nity of forming a mature judgment as to their soundness. 
Volcanic rocks, or rather rock species, are commonly arranged 
under the two classes, Trachytic and Pyroxenic, names proposed 
by Bunsen as the equivalents of acid and basic, and it is hardly 
necessary to observe that when the mineralogical and chemical 
natures of rocks are to be compared, some such classification 
must be taken into account, since it would be as absurd to liken 
a trachytic to a pyroxenic rock as chalk to cheese; it must also 
be remembered that the same volcanic cone may emit lavas of 
both these classes, a fact observed by the Scientific Commission 
at the eruption of Santorin, when in exactly similar manner to 
many ancient outbursts, the trachytic preceded the subsequent 
and more abundant pyroxenic lavas. As regards the mineralo- 
gical and chemical constitution of unaltered volcanic rocks, 
nothing is more certain than that from whatever part of the 
world they proceed, they are essentially made up of a very 
limited number of mineral species, always the same, and the 
application of the microscope to petrology has now proved this 
to be the case also, when they are of so compact a texture as not 
to admit of their constituent minerals being distinguished by the 
naked eye. The examination of any large collection of volcanic 
rocks cannot fail to impress the observer with the wonderful 
similarity of the various rock specimens from one volcano to 
corresponding ones from others situated at the greatest dis- 
tances ; and ample evidence of this may be seen in the writer’s 
extensive collection, the result of many years’ labours in the 
voleanic districts of Europe, America, Australasia, and Africa, 
and in which, for example, specimens may be seen of trachytes 
from Auvergne, the Rhine, or fhe Andes, undistinguishable 
from one another when placed side by side, other lavas from 
Otaheite (where, however, Pele’s hair is not found, as men- 
tioned by Mr. Mallet), to all appearance identical with those 
from Etna, both of which voleanic districts he has had good 
opportunities for studying. 
Pele’s hair, from Hawai, in the Sandwich Islands, so called 
from its having been blown by the action of the winds over the 
surface of the molten lava into hairlike filaments resembling 
spun glass, is simply pyroxene, a mineral which, next to felspar, 
is the most common constituent of the lavas of all volcanos. 
When, however, Mr. Mallet asks, ‘* Are the ancient basalts 
and trachytes identical with the modern ones or with each other 
in different localities?” the answer to the first question is 
simply no, for the results of modern petrological inquiry tend to 
show, although no sharp line can be drawn, that the volcanic 
rocks which made their appearance in the successive stages 
through which our globe has passed, although more or less 
characteristic of the epoch, were analogous to, but not abso- 
lutely identical with, those which either preceded or succeeded | 
them ; and to the second question the reply is, that they are | 
| 
identical in mineralogical and chemical constitution, and often 
even approximate closely in percentage composition. 
The well-known researches of Bunsen on the volcanic rocks of 
Iceland, followed up by those of Abich on those of the Cau- 
casus, showed the simplicity and identity in chemical consti- 
tution of volcanic rocks, and the later results of trustworthy 
chemical analyses, not of fragments chipped off at random, but 
of such as represent the mass of the unaltered rock itself, are 
every day bringing forward more complete evidence of this 
being the case; this is, without doubt, well understood 
by Professor Palmieri, for the very words cited by Mr. 
Mallet, in which he mentions that ‘‘two specimens of the 
same lava appear indeed to have their constituents in dif- 
ferent proportions” are qualified by stating that the observa- 
tory did not possess the means of arriving “‘at any conclusion ” 
on this point, and by expressing the hope that Prof. Fuchs, who 
had specially devoted himself to this subject, would, by employing 
‘*well selected and sufficiently large specimens,” obtain satis- 
factory results. Not only do all chemists and mineralogists know 
that there may exist a co siderable difference in the percentage 
composition of mineral species which are of identical chemical 
constitution, but in answer to Mr. Mallet’s questions as to iron 
blast-furnace slags, every scientific metallurgist will admit that 
the basis of good smelting, necessitates the production of slags 
having a constant and definite chemical constitution, and that 
not only should the slag from every two tappings|be identical in this 
respect, but that so long as the furnace works properly, and the 
same materials are charged into its mouth, the same slag will 
also flow from its hearth ; many yeais’ practical experience in the 
management of blast-furnaces, and the numerous analyses of 
slags which I have made, some of which will be found published 
as far back as 1846 in the British Association reports on 
the crystalline slags, have not only fully satisfied me on this 
point, but shown me examples of iron blast-furnaces, which, 
from their having been constantly fed with precisely the same 
ore, fuel, and flux, have not only for successive tappings, but for 
years, produced slags, not only identical in chemical constitution, 
but in which the percentage of the constituent silica and bases 
have only varied within extremely small limits, 
When Mr. Mallet, however, asks such questions as whether 
the crystalline minerals of volcanic rocks are identical, and 
furnishes in his critique the most ample evidence of his con- 
founding chemical constitution with percentage composition, 
ignoring altogether the laws of isomorphism and the substitution 
of bases, I believe mineralogists will absolve me from taking up 
more space in discussing further these questions. The implica- 
tion of being unacquainted with the works of yon Waltershausen, 
Senft, Blum, and my good friend Zirkel, which have been 
in my hands, I might almost say warm from the press, is 
easily disposed of, as numerous references will be found to them 
in my published papers; curiously enough, not long back I 
referredin a paper to opinions of von Waltershausen which are 
diametrically opposed to those held by Mr, Mallet; in the 
Geological Magazine for 1867, p. 227, references will be found to 
the other three works. If Mr. Mallct’s knowledge of recent 
petrology is based upon ‘‘Blum’s Handbuch der Lithologie,” 
which he recommends “above all,” I would remind him that 
this work, although a very excellent one when it was written in 
1859, is now quite antiquated, this branch of mineralogica 
science being then, as it were, only in its cradle as compared 
to the great advances which have been made during the last 
eight or ten years: “von Waltershausen vulkanische Gesteine” 
appeared still earlier, in 1853. 
T would remark that neither in this communication, nor in my 
review, was it the intention to take into consideration Mr. 
Mallet’s theory of volcanic energy, and it was only alluded to 
because, in his introductory sketch, he so altogether overlooked 
those explanations which, notwithstanding his reply, will still be 
demanded by chemists, mineralogists, and geologists, before 
they can accept his views ; I still object most strongly to the 
tone and style of his introductory sketch, and I am not alone 
in doing so. 
Thornton Cottage, Aug 8. Davip Fores 
Explosion of Chlorine and Hydrogen 
Some time ago, being desirous of showing a class the explo- 
| sion of chlorine and hydrogen by artificial light, I devised a 
simple method which was perfectly successful., Equal volumes 
of the two gases, prepared separately by the usual methods, were 
