392 
NATURE 
| August 13, 1885 
perspective causes—viz. by the points of the streamers 
being further distant than the bases. It is the same per- 
spective peculiarity which causes the lamps in a street or 
the trees in an avenue to appear to meet in the distance. 
“ The centre of the corona is sometimes dark—that is to 
say, the sky is seen between the streamers, at other times 
the central part is filled with luminous matter. 
“Tt is not only the streamers which contribute to form 
the corona ; on the contrary, all the forms of the aurora 
lend their beauty to produce this magnificent display. If 
to this is added that the Aurora Borealis in such moments 
develops its greatest strength, richest colour, and most 
intense light, it will be understood that the corona is that 
form of the phenomenon which possesses the greatest 
magnificence and most striking beauty.” 
With regard to the height of the aurora, a preliminary 
examination of the observations made in the plane Kouto- 
kaeino-Bossekop gives from 50 to 100 miles, an average of 
18 measurements giving 70'2 miles or 113 kilometres. 
From this long article on auroree, the reader must not 
think that our author is exclusively occupied with them. 
His two volumes are admirable examples of what books 
of travel should be, and it falls to the lot of few travellers 
to have such an interesting region to explore, or to have 
such an important piece of scientific work to accomplish. 
THE BRITISH ASSOCIATION 
UR readers are aware that at the approaching meet- 
ing of the British Association it has been arranged 
to have discussions in Section A on kinetic theories of 
gases and on standards of white light. Prof. Crum 
Brown has consented to open the discussion on the 
kinetic theories, and has drawn up the following. short 
abstract of points to which he proposes to allude. It 
would be convenient if persons desiring to take part in 
the discussion would forward their names, with, if possible, 
a short abstract, to the recorder, Prof. W. M. Hicks, Firth 
College, Sheffield. 
Difficulties connected with the Dynamical Theory of 
Gases.—Prof. Crum Brown. 
The Dynamical Theory of Gases appears at first sight 
to furnish a very complete explanation of all the pro- 
perties of gases, both physical and chemical. When, 
however, we come to details, difficulties and apparent 
contradictions make their appearance. These difficulties 
have been pointed out from time to time, and some 
attempts have been made to show that they are not really 
fatal to the theory as usually stated ; but it may be useful 
that some of them should be brought at this time before 
the section and regularly discussed. 
I shall here merely mention some of these difficulties, 
as the explanations which have been given of them will be 
better supplied by others in the discussion. 
1. The difficulties connected with the doctrine, that 
energy communicated from without to a gas ts equally 
shared among the whole of the degrees of freedom of the 
molecules. This leads to a relation between the numbers 
of degrees of freedom and the ratio of the specific heat at 
constant pressure to that at constant volume. This ratio 
is for mercury gas almost exactly 5:3, from which it would 
appear that the molecules of mercury gas have not more 
than three degrees of freedom—in other words, that the 
whole energy of mercury gas is kinetic energy of trans- 
lation of the molecules. But even if we assume that the 
molecules of mercury are spheres, perfectly smooth and 
perfectly rigid, the fact that mercury vapour has a spectrum 
points to some form of energy of a vibratory kind. Again, 
the gases, the molecules of which are supposed to con- 
sist of two atoms, have the ratio of the specific heats 
nearly equal to 7:5 (it seems always to be a little greater 
than this, which increases the difficulty). This points to 
five degrees of freedom of the molecule, which would be 
consistent with the hypothesis that these molecules con- 
sist of two smooth, undeformable spheres at a constant 
distance from each other, the five degrees of freedom 
being three of translation and two of rotation about two 
axes, any two at right angles to each other and at right 
angles to the axis of the molecule, that is, the line joining 
the centres of the two atoms. But here also we have 
spectra, and in addition the phenomena of dissociation 
lead to a belief that the firmness of the union of the two 
atoms diminishes as temperature rises, and it is difficult 
to reconcile this with a constant distance of the two atoms 
from one another in the molecule, Any variation in this 
distance would be a new degree of freedom in addition 
to the five allowed by the theory. 
All attempts to reconcile chemical action and chemical 
equilibrium with dynamical conceptions seem to require 
the assumption of vibrations of the atoms in the molecule, 
under the influence of forces depending on the distances 
of the atoms from each other, and perhaps in addition to 
these, vibrations of the atoms as parts of the molecule, 
vibrations of the atoms themselves. In molecules, even of 
a comparatively simple kind, such considerations imply 
many degrees of freedom, certainly far more than the 
dynamical theory of gases as usually understood will 
admit. 
2. Difficulties connected with the doctrine that energy 
of each kind ts distributed among the molecules according 
to some form of the law of probability. 
This implies that in a gas at any temperature there are 
molecules in the condition as to energy which is the 
average condition of the gas at any other temperature. 
That, for instance, at the ordinary atmospheric temperature 
there are molecules in the condition which is the average 
condition at a red heat. 
This seems inconsistent with what is usually regarded 
as true, viz., that there are limiting conditions of tempera- 
tureand pressures, on the one side of whichcertain chemical 
changes occur, while they do not occur at all on the other 
side. Thus at ordinary atmospheric temperatures and 
pressures, hydrogen and oxygen show no tendency to 
combine. Ata red heat they combine almost completely. 
At ordinary temperatures phosphorus combines slowly 
with oxygen if the pressure of the oxygen is below a cer- 
tain limit (dependent on the temperature), but apparently 
not at all if the pressure of the oxygen is above that limit. 
Many other cases might be mentioned, but these may 
suffice as instances. It is difficult to understand the 
existence of such definite sharp limits, if the energy is 
distributed among the molecules according to any 
asymptolic law. In such a case the rate of chemical 
action might be expected to diminish, but not to become 
zero. 
I have brought forward these instances of apparent 
contradiction between the conclusions of the dynamical 
theory as usually stated, and observed facts in the hope 
that they may be cleared up. This may conceivably be 
done in two ways—either by showing that the facts have 
not been accurately observed, or that the conclusions 
have not been legitimately drawn from the theory. 
NOTES 
THE Iron and Steel Institute holds its summer meeting in 
Glasgow on September I-5. The programme includes excursions 
down the Clyde and a visit to the Forth Bridge Works. The 
following is the list of papers down for reading :—On the iron 
trade of Scotland, by Mr. F. J. Rowan, Glasgow ; on the rise 
and progress of the Scotch steel trade, by Mr. James Riley, 
Glasgow, Member of Council; on the present position and 
prospects of processes for the recovery of tar and ammonia from 
blast furnaces, by Mr. Wm. Jones, Langloan Ironworks, N.B. ; 
on the structural features and working of the South Chicago 
blast furnaces, by Mr, F. W. Gordon, Philadelphia, and Mr. 
