developed within relatively quite recent periods. The great heat that has prevailed throughout the greater part of the history of the solar system, and which, indeed, still prevails in its nucleus, the sun, which is still 991315 (99⋅866) per cent, of the entire mass of the system, or practically the whole of it, has prevented the formation of any of the substances which we know to be composite. It is only in the comparatively minute masses which have been accidentally separated from the rest, and which, in consequence of their diminutive size, have earlier reached the point at which the radiation exceeds the generation of heat, that conditions have been produced under which these comparatively unstable substances, such as water, carbonic dioxide, and the other oxides comprising the earth's crust, could exist. In proportion as the degree of heat diminished, the capacity for more and more unstable substances increased. The earliest compounds were those in which silicon, potassium, sodium, magnesium, etc., combine with oxygen, several of which were, from their great stability, long regarded as elementary. Then came a variety of acids, alkalies, and salts, together with compounds of the metals. Later, as the temperature still further lowered, the oxygen was enabled to seize the hydrogen and form the gaseous protoxide, steam, which at a still later period, when the temperature of the earth's surface fell below 100° Centigrade, condensed into water. Long prior to this, carbonic acid had been formed, and, doubtless, constituted at that time fully one half of the earth's atmosphere. The vast amount of free carbon now existing in the earth, and, still more, that which is fixed in the chalk and limestone formations, all of which must have formerly existed in the atmosphere in the form of carbonic-acid gas, indicates that the above estimate is probably far too low.
All the compounds thus far referred to, and all others having a certain degree of stability, must have been first formed at a period of considerable heat, the dissociation point of all compounds having been estimated at 6,000° Centigrade; although this, doubtless, varies for different compounds as greatly as do the condensing points of different gases. But there are, besides, many compounds which are continually forming at such temperatures as now prevail on the surface of the earth, and most of these are very much more unstable than those last mentioned. The elements which chiefly enter into such compounds are oxygen, nitrogen, hydrogen, and carbon, all but the last named of which are gaseous at ordinary temperatures. The substances of this nature with which we are familiar are known as organic compounds, and such as we see are, in fact, the products of organized beings from the different parts of which they are obtained. But this process should not be regarded as any less cosmical than that by which the rocks or the metals have been evolved out of primordial matter.
Time forbids the further following out of this series of steps in the development of existing forms of matter, but it will be readily per-
