Page:Popular Science Monthly Volume 34.djvu/870

848 an observer of solar phenomena, the chapters on the sun are among the most interesting, instructive, and generally valuable portions of the book. It is worth while to quote two or three passages from these chapters in which he sums up the latest results of investigation and theory. After quoting Secchi's later eruption theory of sun-spots, and pointing out an obvious objection to it, he remarks: "Perhaps the true explanation may be that when an eruption occurs at any spot the photosphere somewhere in the neighborhood settles down in consequence of the diminution of the pressure beneath, thus forming a sink, so to speak, which is of course covered by a greater depth of cooler vapors above, and so looks dark"(p. 190).

In regard to the disputed question of the influence of solar disturbances, as indicated by sun-spots, upon the meteorology of the earth, concerning which some extravagant notions have recently been set forth. Prof. Young says, "It is now quite certain that whatever influence the sun-spots exert upon terrestrial meteorology is very slight, if it exists at all." This statement, coming from one who ranks so high as an authority on solar physics, must be rather discouraging to those who have been trying to fix the responsibility for every great tornado, or other unusually destructive storm of late years, upon some unfortunate sun-spot.

Our author speaks with characteristic modesty, and yet very interestingly, of the phenomenon of "the reversing layer," first observed by him during the solar eclipse of 18*70, and which seems to indicate the existence of a gaseous stratum or shell surrounding the photosphere, and not above five hundred miles in thickness, to which the formation of the dark lines in the solar spectrum is mainly due. While by no means abandoning his own opinion of the probable nature of this phenomenon, he frankly states the opposing view of Mr. Lockyer, and points out how observation may be directed to settle the question.

A most inspiring and encouraging statement for those who may be troubled by doubts as to whether any important discoveries remain to be achieved by future students of the sun is that "among the many thousand lines of the solar spectrum only a few hundred are so far identified." There are twelve elements familiar to us on the earth, which are known to exist in the sun, and nine others of whose existence there the evidence is not quite conclusive. Prof. Young does not pronounce quite so positively as some foreign savants have done against the validity of Dr. Henry Draper's conclusion that his photographs had demonstrated the presence of oxygen in the sun, but he remarks that the latest work appears to turn the balance of evidence the other way. He still accepts Rosetti's determination of the effective temperature of the sun, 18,000° Fahr., as being the most probable that has yet been obtained.

The extraordinary mental picture that we must form of the solar globe, as a body in the gaseous condition and yet possessing in its nuclear mass a consistency like that of tar, has become familiar to readers of the literature of science since the publication of Prof. Young's admirable book on "The Sun," In the present work he presents in briefer form the same general conclusions concerning the constitution of the sun. There are few who will be disposed to accept by preference the views of those who hold that the great mass of the sun is probably liquid instead of gaseous. The brief synopses given of our knowledge of the nature of the visible phenomena of the sun are exceedingly clear and succinct. To begin with the photosphere, which, as the reader knows, is the visible surface of the sun, from which the splendor of its light arises: "The photosphere is probably a shell of incandescent clouds, formed by the condensation of the vapors which are exposed to the cold of space.

"The photospheric clouds float in an atmosphere containing, still uncondensed, a considerable quantity of the same vapors out of which they themselves have been formed, just as in our own atmosphere the air around a cloud is still saturated with water vapor. . ..

"The chromosphere and prominences are composed of the permanent gases, mainly hydrogen and helium. . . . The appearances are for the most part as if the chromosphere was formed of jets of heated hydrogen ascending through the interspaces between the photospheric clouds, like flames playing over a coal-fire.

"The corona also rests on the photosphere, . . . but extends to a far greater