296 
THE GEOLOGIST. 
material. The thermal portiou of space must therefore be material that 
is occupied by matter. There can be no absolute void or pure space be- 
tween us and any visible star, no matter how remote ; even thouj^n its dis- 
tance be so immense that the courier ray that now announces to our senses 
its existence, may have started on its errand thousands of millions of years 
ago, travelling all that time at the rate of a hundred and seventy thousand 
miles a second, still in the immense, the inconceivable space so run over 
by the luminous messenger, no absolute void could have existed, otherwise 
the star must be invisible to us. 
Mr. Mackie is therefore right in stating "that if heat be confined to 
matter, how can we speak of hot and cold regions of space ? " but he 
should have added pui'e or empty space where no matter exists, as I only 
meant those portions of space occupied by matter surrounded by and float- 
ing in a thermal ocean ; for whether heat be a fluid or a wave, we cannot 
expect to find the causes of telluric changes of temperature by travelling 
into regions of space where nothing exists, where there is no entity but 
nought, " where death is life." Mr. Mackie says (last number of ' Geo- 
logist'), "if the sun moves on with his surrounding worlds, these will all 
travel onwards together in the same ethereal material envelope ; and there- 
fore, unless the supposed hot and cold regions of space have temperatures 
of much higher or much lower degrees than the general temperature of 
the solar region, the effect would be imperceptible." It is evident that 
the portions of space so traversed by our solar system at the rate of 57^ 
miles a second, moving towards the constellation Hercules, must be ma- 
terial, for so far as we can see any object in the universe, between us and 
that object there can be no absolute void or pure space, otherwise it must 
be mvisible. Kow it is clear that the regions of space occupied by matter 
cannot be of the same temperature, as the causes that generate light and 
heat are neither uniform in intensity nor distance. The path traversed by 
our solar system in space therefore cannot be isothermal. 
It is not likely that our solar system is travelling through space sur- 
rounded by the same ethereal envelope, as Mr. Mackie seems to think, for 
this would be, supposing that outside this ethereal envelope nothing ex- 
isted but pure space, an assumption quite opposed to the facts and reasons 
already stated. Besides, whether heat be a fluid or a wave, in either case 
it must be subject to the ordinary influences of physical agencies ; there- 
fore the same condition of matter constituting unifoiTnity of temperature 
could not follow and surround our solar system in its travels through 
space. 
David Leslie, M.D. 
Tunhridge, July \m, 18G3. 
The Portland Fissures. 
Sir, — I liope you will allow me to correct a mistake which you have 
made in your remarks on my last letter, in saying that my theory was 
tliat of " the deposition of the extinct animals in caves before the caves 
existed." On the contrary, I said that I was of opinion that all bone- 
caves were only formed by the animal remains embedded in the limestone 
deposit before its consolidation, and, consequently, before the existence of 
any caves in it. 
The question with respect to the Portland and Oreston fossils is en- 
tirely dependent on the truth of certain facts. First, with respect to 
the Portland fossils, can the statement of Captain Manning, of the ' Willis's 
