origin; in another, illustrated by 19 Piscium, they have been definitely identified as of carbon origin. The relation between the two types is not clear. It has even been advocated that the evolutionary process divides shortly after passing the solar stage; that the reddish stars with absorption bands sharply terminated on the violet edges are on one branch, and that the very red stars with absorption bands sharply defined on the red edges are on the other branch. This plan of overcoming a difficulty seems to me to introduce a greater difficulty; and I do not doubt that systematic investigation will supply the connections now missing. That the denser edges of the bands in Type IV (Secchi) should occupy the same positions as the denser edges of absorption bands in Type III, can hardly be without significance; and Keeler's view that the carbon absorption bands in Type IV are matched by carbon radiation in some stars, at least, of Type III suggests a most promising line of investigation for powerful instruments.
There is scarcely room for doubt that these types of stars are approaching the last stages of stellar development. Surface temperatures have lowered to the point of permitting more complex chemical combinations than those in the sun. The development of 'sun spots' on a large scale is quite probable, and the first struggles to form a crust may be enacted. Type III includes the several hundred longperiod variable stars of the Omicron Ceti class, whose spectra at maximum brilliancy show several bright lines of hydrogen and other elements. The hot gases and vapors seem to be alternately imprisoned and released. It is significant that the dull red stars are all very faint;—there are none brighter than the 51⁄2 magnitude. Their effective radiating power is undoubtedly very low.
The period of development succeeding the red-star age of Type IV has illustrations near at hand, in the planets Jupiter and the earth; invisible save by borrowed light. When the interior heat of a body shall have become impotent, the future promises nothing save the slow leveling influence of its own gravitation and meteorological elements. It is true that a collision may occur to transform a dark body's energy of motion into heat, sufficient to convert it into a glowing nebula, and start it once more over the long path of evolution. This is a beautiful theory, but the facts of observation do not give it satisfactory support. There is little doubt that the principal novas of recent years have been the results of collisions, either between two massive dark bodies, or between a massive body and an invisible nebula. The suddenness with which intense brilliancy is generated would seem to call for the former, but the latter is much more probable, in view of many facts. The nebular spectra of the novas are generated in a few months; but in every case thus far observed the bright nebular bands grow faint very rapidly, and in the course of a few years leave a continuous spectrum, apparently that of an ordinary star. Either the masses
