486 
volcanic ejecta, by the analogy of the blast furnace, in which 
the same materials in the same proportions do not even in any 
one furnace, or at all times, produce identical slags. 
What is Mr. Forbes’s reply? That the zzdention of the iron 
master is to produce slags always the same, as the indication that 
the furnace is working well. 
Doubtless the intention and desire of the iron-master is to pro- 
duce good iron, and at all times as nearly as he can such a slag 
as indicates that he is doing so, But, asa matter of fact, he is not 
able to reach this. He can only approximate to constancy in 
the chemical or mineralogical constitution of his slags, which are 
never identical, even for short periods, Is this substitution of the 
intentions of the iron-master for the actual facts of the blast fur- 
nace slags, on Mr, Forbes’s part, worthy of the candour of the 
searcher for truth ; or does it not rather resemble the dialectic 
wriggle of the advocate? 
Complete identity between any two rocky masses, ejected or 
otherwise, can only exist where the same elements in the same 
proportions are combined inthe same way, and in the same 
molecular aggregation. If the mere presence in greater or less 
Proportion in the mass, of certain crystallised minerals in any 
variable proportion, such as felspar, pyroxene, or leucite, in the 
magma of lavas, were enough to constitute identity, then nearly 
all the known rocks of ‘the world, crystalline, igneous, and sedi- 
mentary, might be viewed as identical, for all consist of a few 
elements and of a few prevailing simpler minerals, 
While still seeming to maintain his original statement, Mr. Forbes 
nowsubstitutes forzdentity—a great similarity in all volcanic rocks, 
Further discussion is therefore needless—nor indeed would discus- 
sion of my views as to volcanic heat, &c., lead to ‘any good result— 
with a gentleman whose notions of scientific method are such, 
that after six months’ consideration he holds} any distinction 
between hypothesis and theory to be mere hair-splitting, and 
whose notions of physico-mechanics are of that confused cha- 
racter, that he views pressure and work to be quite the same, 
and that it is matter of indifference whether we talk of “ pressure 
conytrted into its equivalent, heat,” or of work transformed into 
eat. 
Would Mr, Forbes enlighten your readers by stating in figures 
what is the equivalent in heat, of the pressure _of a weight of ten 
pounds, resting upon arigid level plane? 
Were Mr. Forbes of any real authority upon volcanic subjects, 
‘there might have been more ground for his sweeping and antici- 
patory condemnation of my views as to volcanic energy, which, 
however, in that case, he would never have uttered ; but on looking 
down the list of his published papers, I do not find any treating of 
vulcanology simply, nor am I aware that he thas ever enlarged 
the boundaries of our knowledge in that department by a hair’s 
breadth. 
Mr. Forbes appears to think that chemists, mineralogists, and 
geologists are the sole arbiters of all questions as to the nature 
and origin of volcanic heat and energy, Whatever they may have 
done to add to our knowledge of the visible and tangible pheno- 
mena of volcanic vents or cones, they have as yet contributed 
really nothing to discovering the nature and origin of volcanic 
heat itself, if we except some valuable negative evidence drawa 
from the gaseous emanations by chemists of late years, subver- 
sive of the older theories of the chemical origin of volcanic heat, 
still not quite extinct. It is much more to the physicist and 
theoretic mechanician dealing largely with the physigue du globe, 
that we must look for further light, and whose province it will be 
to decide when the right key shall have been found to that enigma 
of ages, the true nawure and origin of volcanic heat and energy. 
Iam done, sir, with this controversy, unwillingly entered upon, 
not in irritation, as Mr, Forbes states, but because I felt justified 
in protesting against new and I believe important views being 
obscured 2x dimine, by objection based only on error. 
My paper containing those views will ere long be before the 
world. My 100 separate copies (as author) from the ‘ Phil, 
Trans,” are already in the hands of or on the way to many men 
of science. The volume itself of the ‘ Transactions” will no 
doubt appear before the end of the year, and to the verdict of 
the real men of science of the world, versed in the subject and 
competent to judge of it, I leave the result. 
London, Oct. 6 ROBERT MALLET 
On the Equilibrium of Temperature of a Gaseous 
Column subject to Gravity 
From Mr. Clerk-Maxwell’s reply to my note on this subject 
which appeared in yom columns a short time since, it would 
NATURE 
appear that he does not profess so much fully to explain the 
difficulty suggested by me as to show that it is capable of ex- 
planation, referring your readers to his other works for further 
information. I would not, therefore, have troubled you further — 
on the subject had it not occurred to me on reading Mr. 
Maxwell’s letter that I could state the case in such a way as to 
render clearly apparent the ground§ for taking different yiews on 
this point. 
Let a vertical column of gas, subject to gravity and in a state 
of equilibrium as to pressure and temperature, be divided by a 
horizontal plane P into two parts, A above and B below. 
In the time A?# let a mass M, of particles pass in their free 
course from A to B, and amass M, from B to A, 
Let the portion of A from which the particles composing M, 
proceed be called the upper stratum, and the corresponding 
part of B the lower stratum, then the following consequences 
may b2 deduced :— 
I. From the equilibrium of density 
M,= M, 
2. From the equilibrium of temperature the amounts of work 
in M, and M, while passing through P are equal. 
3. From the effect of gravity the work in M while in A 
reckoning from the commencement of the free course of each 
particle composing My, is less than at P, while that in M, is 
greater, . 
4. Whence it follows that of the two equal masses M, and My 
in the upper and lower strata respectively M, contains less work 
than Mg. 
5. The work in M, while in the upper stratum reckoned as 
before, is the same as that of any other equal average mass in 
that stratum, and the same is the case also of Mz. 
6. The average amounts of work in equal masses in the two 
strata, and the consequent temperatures of the strata are unequal, 
the lower stratum having the higher temperature. 
I suppose Mr. Maxwell would deny the truth of statement (5). 
I presume he would argue as follows :— 
“Of all the particles in the lower stratum which in the time 
Athave at the commencement of their free course a velocity 
and direction such as would take them through P, gravity in 
selecting those which compose Mp excludes those whose veloci- 
ties are insufficient to overcome the effects of their weights, while 
in forming M, particles of low velocity are selected (included ?), 
which, but for the effects of gravity, would not have cut P in 
their free courses, consequently the particles in M, have an 
average velocity less than that of the upper stratum from which 
they come, while the particles of M, have a greater average 
velocity than that of the lower stratum, and consequently the 
inequality of the average velocity of the particles in the two 
Strata cannot be inferred from the inequality of the average velo- 
cities of the particles composing M, and M, while in those 
strata,” 
This argument, therefore, assumes the theory that ina given 
mass of uniform temperature there are particles moving with 
every velocity from nothing upwards to a certain limit, and 
mixed in certain proportions. ‘That this is actually Mr. Max- 
well’s view I own I might have remembered, but I suppose I 
overlooked it from an impression in my own mind that the 
molecular motion was to be regarded as being of a planetary (or 
in the case of gases a cometary) nature. That in masses of the 
same temperature velocities were to be regarded as practically 
uniform, except in so far as affected by the distance of the par- 
ticles apart, and that the so-called impacts of particles were 
more properly to be regarded as perihelion passages of bodies 
moving among each other in hyperbolic orbits. If this view is 
the more accurate one, then obviously the argument which I 
have assumed that Mr. Maxwell would use, falls to the ground, 
Is there no possibility of testing the nature of the thermal 
equilibrium of a column of still air? The result would at any 
rate throw an unexpected light’on the nature of molecular 
motion. 
Graaff Reinet College, July 19 F, GUTHRIE 
The Sphygmograph 
Dr. GaLABIN, in his letter published in your last number, 
criticises my explanation of the cause of the small wave in the 
first part of the sphygmograph trace, which he calls the tidal 
wave, In his criticism he does not take into consideration the 
hmodromograph traces of Chauyeau, on which my explanation 
