292 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 11, NO. 13 



namely that the nebula at first was relatively cool, that the tempera- 

 ture gradually rose to that of the hottest stars, and then regularly de- 

 clined to the end of the series, the solid planet. A gaseous mass, con- 

 tracting under the influence of gravitation, became warmer; at its 

 center where the pressure was greatest, the increasing condensation 

 generated still higher degrees of temperature, until a luminous nucleus 

 was formed. As condensation went on with increasing intensities of 

 pressure, the temperature continued to rise until the heat generated 

 by compression was less than that lost by radiation into space, when 

 cooling began. Although this hypothesis, in its crude form, is not 

 universally accepted, it nevertheless gives a fair conception of that 

 part of the evidence with which we are now concerned. The process 

 of evolution from cool to hot and then to cool again is fairly outlined. 

 The nucleus of the original nebula has its modern representative in the 

 Sun. 



In all the foregoing discussion it has been tacitly assumed that the 

 nebula from which the solar system was developed was similar in all 

 essential respects to the planetary nebulae. The latter, as shown by 

 their spectra, consist mainly of hydrogen, helium, andnebulium, with 

 slight traces in some of them of carbon, nitrogen, and perhaps other 

 elements. NebuHum is known only from its lines in the spectrum, and 

 its atomic weight has been estimated by Fabry and Buisson as 2.7, 

 placing it between hydrogen and helium. In any further study of 

 relations between the atomic weights of the elements, nebulium must 

 be taken into account, and perhaps also coronium, so called from its 

 lines in the spectrum of the solar corona. From its position in the 

 corona it is assumed to be lighter than hydrogen, and so would seem 

 to be an even more primitive element. That possibility cannot be con- 

 sidered here; we must limit ourselves to the conditions actually seen 

 in the nebulae. No assumption is made as to the possible ancestry of 

 the nebular elements. They are the visible beginnings. 



Now it is easy to see that in the process of evolution from nebula 

 to sun an orderly development of the elements could hardly have been 

 possible. With changing temperatures, changing pressures, and 

 changing environments all the conditions required for a regular pro- 

 gression according to the order of the atomic weights were lacking. 

 The order actually followed was that of relative stability. In the 

 hotter stars only the most stable elements were formed, and naturally 

 in the greatest abundance. Calcium (atomic weight 40) and iron 

 (atomic weight 56) were among the earliest to appear, while the others, 



