GASES IN INTERPLANETARY SPACE.
The question of whether interplanetary and interstellar space is a vacuum or contains matter in an exceedingly attenuated form is an interesting problem, and one upon which there has long been much speculation. On the one hand the planets give no evidence of an impeding friction, but on the other hand the evidence of such friction in the case of certain comets seems possible. When the earth's atmosphere was supposed to consist solely of oxygen, nitrogen and carbon dioxid, it appeared very improbable that these should pass to any considerable distance away from the surface of the earth, but, with the more recent knowledge of the constituents of the atmosphere, this thesis seems less certain. The discovery of argon by Rayleigh and Ramsay has led to the further discovery of the presence of helium, neon, krypton and xenon, which have enriched chemistry with a new type of chemical element, having no affinity, forming no compounds, and being, as far as has yet been found, perfectly inert. At the same time comes the knowledge that no inconsiderable quantity of hydrogen is a constant constituent of the atmosphere. This has been abundantly proved by Gautier, by Dewar and by Ramsay. Of these gases, hydrogen is the lightest of terrestrially known elements, but helium is not far behind it, and has not yet been changed from its gaseous form to that of a liquid. The methods which have availed to condense hydrogen to a liquid have thus far failed with helium. Turning to the chemistry of the sun, the spectroscope shows the presence of an atmosphere largely of hydrogen, but helium is also present, extending far out from the central mass of the sun. The same instrument reveals lines indicating other elements at still greater heights in the sun's atmosphere, among them one which has been named coronium. From its position far away from the surface of the sun, it seems probable that coronium has a density far less than that of even hydrogen. Again, the evidence of the spectroscope upon the lightest constituents of our atmosphere points to the presence of other gases than helium and hydrogen, and this is reinforced by what the same instrument shows of the aurora. The latter appears to be an electric phenomenon, concerned with elements at least in part now unknown to us, and at a height above the surface of the earth at which it was long supposed there could be no appreciable atmosphere.
It thus appears that the upper strata, both of the sun and of the earth, consist of the lighter constituents which are largely removed from the lower atmosphere by their lightness, and no limit can be placed upon the distance to which these elements would travel from sun or earth into interplanetary space. What is true of sun and earth is doubtless true also of other planets and other suns, and it seems not impossible that even interstellar space may contain these and similar gases in an almost infinitely attenuated condition. What the condition of these gases may be at the temperature of interstellar space, which cannot be far removed from absolute zero, it is difficult to say. On the one hand, at such a temperature they might be expected to be solids, but, on the other hand, the particles would be relatively so few and far apart from each other that they would have the properties of a gas. The great advance in our knowledge of these fields during the last few years gives promise of much new light in the near future.
THE DESTRUCTION OF FORESTS BY FIRE.
A press bulletin from the Bureau of Forestry gives an abstract of a forthcoming paper entitled 'Forest Fires,' by Mr. Alfred Gaskill. By impressing the public with some idea of the peril it suffers from forest fires, and the
