August 27, 1885] 
LETTERS TO THE EDITOR 
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Radiant Light and Heat 
I AM sure that all students must be grateful to Prof. Balfour 
Stewart for his exposition in last week’s NATURE (p. 322) of the 
errors and absurdities into which recent scientific men had 
fallen, and out of which they are now groping their way. But 
if it be not trespassing too much on his good nature, may I ask 
him one or two questions in order to further educe his views on 
points which he cannot but have given much thought to, though 
they are points which, without further explanation, some of us 
are liable to misunderstand. We have some of us had the 
‘advantage of being wrong first,” combined with the further 
advantage of thinking ourselves right, but I for one will now 
gladly admit that I was wrong, if I may thereby hope to join 
“‘the generation which is right.” 
The following are the five points I wish to receive help in 
understanding :— 
(a) ‘‘It is absurd to suppose that particles of air are shot . . . 
with a constant velocity of 1100 feet a second.” 
I am disposed to agree ; but am unable to see clearly how far 
this absurdity destroys the validity of the so-called ‘‘ kinetic 
theory of gases,” and of the mode in which sound is considered 
to be conveyed by such a medium, if indeed it is still so 
considered at all. 
(2) ‘‘Can it be thought that hot bodies emit myriads of very 
small particles, which pass through space with the enormous 
velocity of 187,000 miles per second? Or again, is it likely 
that this velocity should be precisely the same for all bodies and 
for all temperatures ?” 
I should say it was highly zzlikely, in fact, that the idea is 
ludicrously absurd. This is a triumphant refutation of the 
corpuscular theory, but I am rather troubled by the thought that 
the argument seems equally to refute the wave-theory, if for 
‘*particles” in the above sentence, we substitute the word 
“‘waves.” I know it is only my stupidity which causes me to 
feel this difficulty. 
Again, it sometimes seems to me that the undulatory theory 
itself requires a good deal of ‘‘ propping up ;”’ and that several 
phenomena—for instance, ‘‘ aberration”—explain themselves 
more easily and simply on the corpuscular. 
(c) In speaking of the ‘‘transmutation of w7sible energy into 
heat,” we are surely justified in calling heat ‘‘ zzz7s2b/e energy ” 
in contradistinction to the other; but, suppose the blow is so 
intense as to make a flash, are we to consider that flash as part 
of the invisible energy which has been ‘‘ created,” or are we to 
consider it a portion of the visible energy which has escaped 
destruction? The notion of a certain quantity of visible energy 
disappearing from the universe at one place, and an equivalent 
quantity of invisible energy being simultaneously created at 
another, is so beautifully simple and satisfying that I am sure 
the process can be made quite clear to any mind of common 
intelligence with a little more trouble. t 
(d@) ‘* This train of thought enables us. . . to assert that 
there is a definite mechanical relation between the amount of 
heat which leaves a hot body as it cools, and the radiant energy 
which accompanies the act of cooling.” 
I fear I am too stupid to understand this sentence. As I read 
it, it sounds like the following :—‘‘ There is a definite mechanical 
relation between the number of people which leave a train as it 
empties, and the number of people who get out of it and go 
away during the act of emptying.” And the paragraph seems 
to go on thus :—“ If, for instance, ten people get out of a train, 
and all of them enter an omnibus so as to be entirely absorbed 
by it, then, while the train has become ten people emptier, the 
emmmibus has gained an equal number and has become ten people 
uller. 
I know that this is absurd, but I am unable to seize the point 
properly, and therefore venture to put my difficulty in this plain 
and outrageous way. 
(e ‘*Radiant heat is physically similar to radiant light, the 
NATURE 
389 
only difference being that its wave-length is greater, and its 
refrangibility less, than those of light.” 
May I ask if it is known ow much greater ‘‘ the wave-length 
of radiant heat” is than ‘those of light”? The modern dis- 
tinction between them is evidentiy so simple and numerical that 
it must be possible to definitely draw the line and to specify the 
exact wave-length which characterises each, or at any rate which 
partitions the one from the other. 
Similarly it would be a help to us students to haye the re- 
frangibility of radiant heat specified and distinguished from those 
of light, too. 
There are one or two other matters concerning which I should 
have been glad of further information; but I will not now 
trespass further upon your space or upon the good nature of the 
professor. A STUDENT 
In reply to the remarks of a student I may state as 
follows :-— 
(2) In the kinetic theory of gases the pressure of a gas is 
regarded as being due to a bombardment by the molectles of 
the gas, and the velocity of sound in any gas can by this theory 
be shown to be definitely related to the velocity with which 
these molecules move about. 
(6) It is no doubt true that the demonstration of ‘‘ aberration ” 
on the corpuscular theory of light is of a simpler nature than its 
demonstration on the undulatory theory, but I have yet to learn 
that the geometrical simplicity of ;a demonstration is always a 
characteristic of truth. The question is rather, Can ‘‘aberration ” 
be shown to be a legitimate consequence of the theory of undu- 
lations quite apart from the mathematical difficulty or easiness of 
demonstration? If the demonstration is va/id its easiness can 
wait. 
(c) While admitting that our nomenclature regarding energy 
is of a temporary nature, I have hitherto confined the term ‘‘in- 
visible energy ” to that kind of energy the motions constituting 
which are on so small a scale and so rapid that they cannot by 
any means be rendered visible. No doubt we see a red-hot 
body, but we do not and cannot see the motions of the individual 
molecules of the hot body. 
(d) The train of thought referred to was that which concluded 
that the particles of a hot body (like those of a sounding body) 
are in a state of vibration and (in both cases) communicate their 
energy of vibration to a medium which surrounds them. It is 
thus a question regarding energy, therefore a mechanical ques- 
tion, and we are thus entitled to assert that there is a definite 
mechanical relation of equivalence in energy between the 
amount of absorbed heat which leaves a hot body as it cools 
and the radiant energy which accompanies the act of cooling. 
We have now so clear and definite a conception regarding 
energy that ‘‘A Student’s” simile of a train and an omnibus 
represents the truth, and it may perhaps look a trifle ridiculous 
to assert such an obvious equivalence. But my remarks were 
partly historical, and to the physical student of a past genera- 
tion the equivalence would not be equally clear. The meaning 
is that the radiant heat and light given out by a body when 
cooling, measured in any way you like and used up in any way 
you like, will always be mechanically equivalent to the amount 
of ordinary heat which the body has lost. 
(e) Your correspondent asks how much greater the wave- 
length of radiant heat is than that of light. Let me refer him to 
a diagram which was given in a recent number of NATURE in 
illustration of a lecture by Prof. Langley, and which will like- 
wise be reproduced in the course of this present series of articles. 
BALFOUR STEWART 
Pulsation in the Veins 
THE writer of a very long and exhaustive article on ‘‘ The 
Heart,” occupying forty-one pages in Rees’s ‘‘ Cyclopedia,” 
quotes, among other authorities, Bichat, who says ‘‘that the 
blood, when it has arrived at the veins, is no longer influenced 
by the heart’s action ; consequently these vessels have no pulsa- 
tion”? . . . ‘‘that the blood’s return in the veins is involved in 
an obscurity ;” and he propounds as a “contrast” ‘‘ the fact of 
general pulsation in the arteries, the absence of this in the 
veins.” The writer of the article states that ‘‘ many authors, 
particularly Haller, considering that this [the venous] system 
has no agent of propulsion, have ascribed to the veins some 
peculiar structure” of which the evidence is insufficient ; also 
“that there is no analogy to the course of the blood in the 
