LIGHT, LUMINARIES AND LIFE. 187 
in recent years by Professor Joly, altogether untenable. I had 
that in my mind when, in my previous paper, I spoke of the 
“diminution of the salinity of ocean waters” during the 
geological ages, as one of the conditions making for advance in 
the evolution of organic life. 
Let us go a step further in the evolution of this planet. 
Owing to its comparative smallness the earth has long since 
passed its solar phase, though it has not yet reached the senile 
condition of the smaller planet Mars. By loss of heat through 
radiation into space, and by concentration uuder the influence 
of gravitation a stage was reached at which this globe consisted 
of a molten ball rotating in space, but for a long period of time 
enveloped in such a dense mantle of vaporous and gaseous matter 
(not water-vapour only), that the radiation of heat from the 
incandescent globe must have been ettectually retarded, owing 
to the low conductivity of the vaporous envelope. Very great 
changes must have occurred during this long-continued “ pre- 
oceanic stage,” as I have called it,* of our planet’s history, 
before the first portions of steam condensed into water upon 
its surface at a temperature much higher than that at which 
water boils under the pressure of our present atmosphere, 
which we measure daily by means of the barometer.t It has yet 
to be shown, [ think, that the “ Crystalline Schists” may not 
have their special characters accounted for by their production 
then through mineral changes in the presence of highly- 
superheated steam ; conditions which would admit of such a 
kind of “sedimentation ” as some petrologists perceive in them. 
The contention of mine more than twenty years ago that they 
represent the first-formed “crust” has since been endorsed by 
such an eminent geolouvist as Dr. Andrew C. Lawson,f the 
Professor of Geology in the University of California. 
Note to [II[.—The paragraph in which I definitely put forward 
the idea of the nucleate origin of the planets runs as follows :— 
“Given a nebulous mass of matter in a state of elemental dis- 
sociation and losing heat by radiation into space, a point must be 
reached, at which condensation of certain elements (those possessed 
of the highest condensation-temperatures and the least potential 
* See Chemical and Physical Studies on the Metamorphism of Rocks. 
The mathematicians like Kelvin and G. Darwin seem to persistently 
overlook this, and the geologists seem to fail to understand it, which is 
not perhaps to be wondered at. 
t See A. Irving (op. cit.) ; also letters to Vature, vol. lxxii, pp. 8 and 79. 
t See Bull. Geol. Soc. of america, March, 1890. 
