August 6, 1885 | 
perfect in this respect it is at any rate more perfect than 
any other substance. 
We can have now a very clear conception of what takes 
place when we heat a body such as coal. _ At first it gives 
out a spectrum consisting of rays, all of which are 
less refrangible than those of the visible spectrum. 
Soon, however, as the coal continues to rise in tempera- 
ture, it not only increases the number of such rays but 
takes on others of a more refrangible nature, entering 
into the visible spectrum when it begins to be red-hot. 
Thereafter it pushes its way further and further into this 
spectrum, taking on successively yellow and green rays, 
blue, violet, and actinic rays as the temperature still rises, 
until at length it shines forth with the lustre of the electric 
light or of the sun. 
Let us now proceed to reply to the fourth question, 
What is meant by a hot body? At first it was supposed 
that heat was a substance possessing mass but not weight, 
an imponderable, as it was termed, which insinuated itself 
between the particles of bodies, thus causing them to 
expand. This substance was further supposed to be 
rubbed out by friction and beaten out by percussion. It 
will be perceived that we have here a corpuscular theory 
of heat very similar to that of light, the one forming 
indeed the natural sequel to the other. The experiments 
of Davy, in which two pieces of ice both below o° were 
made to melt one another by their mutual friction, and 
those of Rumford, made in boring cannon, sufficed, in the 
course of time, to convince physicists that heat cannot be 
a substance, inasmuch as the melting of the ice in Davy’s 
experiments, and the heat produced in those of Rumford, 
would equally imply the creation in large amount of the 
matter of heat. It was therefore concluded by both these 
experimentalists that heat is not a substance but rather a 
_ species of energy. That is to say the only difference 
between a hot body and the same body when cold is that, 
in the former state the molecules of the body are in violent 
motion backwards and forwards, while in the last state 
this kind of motion is much less. This is the dynamical 
theory of heat at present universally held. In it heat is 
regarded as a kind of energy, so that when heat is pro- 
duced by friction or percussion, a certain quantity of 
visible energy disappears from the universe, while at the 
same instant an equivalent quantity of heat-energy appears, 
or is created. 
A little reflection will, however, show us that there is not 
here any 7ea/ creation or annihilation, but merely the simul- 
taneous disappearance of one kind of energy and the appear- 
ance of another ; in fact, nothing more than a transmuta- 
tion of energy. Joule was the first to prove the definite 
mechanical relation that exists between the visible energy 
which disappears and the heat which is generated, and 
according to his experiments, if a pound of water were to 
fall from a height of 772 feet under gravity, and if all its 
visible energy on reaching the earth could at once be con- 
verted into heat, the water would be found to have risen 
1° Fahr. in temperature. It will at once be recognised 
that just as the material or corpuscular theory of heat fits 
into the corpuscular theory of radiant light, so does the 
dynamical, or energetic theory of heat fit into the undula- 
tory or wave hypothesis. We may, in fact, imagine the 
little particles or molecules of heated bodies to be in a 
state of continual vibration resembling in this respect a 
bell, or the string of a musical instrument, except that 
their vibrations are much more rapid than those which 
constitute sound. 
And just as the vibrations of a bell are carried off by 
the gaseous medium, z.e. the air which surrounds the bell, 
and ultimately affect our ear, producing the sensation of 
sound, so are the vibrations of molecules carried off by a 
medium (the ether) which surrounds them and ultimately 
affect our eye, producing the sensation of light. This 
train of thought enables us at once to reply to our fifth 
question, and to assert that there is a definite mechanical 
NATURE 
327 
relation between the amount of heat which leaves a hot 
body as it cools, and the radiant energy which accom- 
panies the act of cooling. And this definite mechanical 
relation may be stated in very simple language. If, for 
instance, a pound of water cools through 10° Fahr. then 
the radiant energy which it gives out in the process of 
cooling, if this should be made to impinge upon another 
pound of water, and be entirely absorbed by it, would 
heat it through 10°, so that while the one pound of water 
has become Io° cooler the other has been raised an equal 
amount in temperature. 
We are now in a position to reply as follows to the 
questions proposed : 
(1) Radiant light consists of an undulatory motion in a 
medium called ether. 
(2) It moves with the velocity of 187,000 miles per 
second. 
(3) Radiant heat is physically similar to radiant light, 
the only difference being that its wave length is greater, 
and its refrangibility less than those of light. 
(4) A hot body is one whose molecules are in rapid 
motion. 
(5) There is an equivalence in energy between the 
amount of radiant light and heat emitted by a hot body 
and the sensible heat which the body loses. Radiant 
light and heat may be termed radiant energy. 
Without pretending to enter here into a philosophical 
discussion it is instructive to notice that all of these 
questions which were capable of being answered in two 
ways were answered wrongly at first. 
Although this procedure of the human mind has de- 
layed the correct solution of a very important series of 
questions, yet we in the present age cannot reasonably 
complain of what has taken place. It has given usa con- 
fidence in our present views that we could hardly have 
had if the question between two alternative views had not 
been threshed out in the past. 
We can thus look to the future without dismay, and 
need not fear the gradual rising into strength of a school 
which shall call in question any of the very important 
conclusions at which we have now arrived. 
Surely there is an advantage in being wrong first and 
right afterwards, especially when it was a past generation 
who went wrong and we ourselves who are right ! 
BALFOUR STEWART 
(To be continued.) 
NOTES 
WE understand that Prof. Huxley, P.R.S., has agreed, at 
the request of the Lords of the Committee of Council on Educa- 
tion, to continue to act as Dean of the Normal School of Science 
and Royal School of Mines at South Kensington, and also to be 
responsible for the general direction of the biological instruction 
therein. 
Tue Senatus of the University of Edinburgh resolved at its 
last meeting that a lectureship of comparative embryology be 
instituted, and appointed Mr. George Brook, F.L.S., as lecturer, 
subject to the approval of the University Court. Mr. Brook has 
for some time been engaged in making investigations for the 
Fishery Board for Scotland. 
THE Indian Civil and Military Gazette writing of the ornitho- 
logical collection presented by Mr. Allan Hume of the Civil 
Service of India to the British Museum, says that its value and 
extent are only now beginning to be realised. It amounts to 
62,000 skins of all kinds, and it has cost Mr. Bowdler Sharpe, 
of the Natural History Department of the British Museum, more 
than three weeks of uninterrupted labour to pack and send it 
away. yen now the work is not at an end, for the collection 
of eggs, which is no insignificant one, remains to be despatched. 
The gift, which represents the labour and learning of a lifetime 
