EVOLUTION OF THE CHEMICAL ELEMENTS. 537 



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 99f| (99-866) per cent, of the entire mass of the system, or prac- 

 tically the whole of it, has prevented the formation of any of the 

 substances which we know to be composite. It is only in the com- 

 paratively 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 com- 

 paratively unstable substances, such as water, carbonic dioxide, and 

 the other oxides comprising the earth's crust, could exist. In propor- 

 tion 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 oxy- 

 gen j several of which were, from their great stability, long regarded 

 as elementary. Then came a variety of acids, alkalies, and salts, to- 

 gether 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, con- 

 densed into water. Long prior to this, carbonic acid had been formed, 

 and, doubtless, constituted at that time fully one half of the earth's at- 

 mosphere. The vast amount of free carbon now existing in the earth, 

 and, still more, that which is fixed in the chalk and limestone forma- 

 tions, 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- 



