118 MEMOIES OF THE NATIONAL ACADEMY OF SCIENCES. 



This neglects no one of the established features of the corona and has apparently contented 

 most writers. It is essentially the explanation given by Professor Young in his work on The Sun, 

 though he does not fail to note grave diiiiculties in the way of accepting it. When, however, we 

 demand of the theory a quantitative agreement with observation, we find that it utterly breaks 

 down. Some of the assumptions involved are so exceedingly improbable that we refuse to regard 

 as plausible a hyi)othesis resting on them, while others equally as essential are absolutely nega- 

 tived by the observations. We will consider these points in turn : 



First («). Since the sijectroscope demonstrates that the gaseous pressure at the limit of the 

 chromosphere is very small, probably far less than that of an inch of mercury, this supposition 

 requires that the pressure of the assumed atmosphere from 000,000 to 1,200,000 miles deep shall be 

 thus inconsiderable, notwithstanding that the force of gravity is more than twenty-seven times as 

 great at the surface of the sun as at the surface of the earth. As improbable as this consideration 

 renders the assumption of a gaseous atmosphere, we find a still stronger objection in the motion of 

 comets. All optical evidence of the existence of our own atmosphere ceases at a height of about 

 4.5 miles ; still, the density at more than twice that altitude is sufficient to offer a resistance to bodies 

 moving with velocities averaging 1.4 times that of the earth in its orbit, such as to render them 

 incandescent almost instantly. Now, the illumination of a particle in the corona is not indefinitely 

 greater than that of one in our own atmosphere, the ratio being that of the angular area of the sun 

 as measured from the two points. It follows, then, that the density of a visible atmosphere near 

 the sun cannot be indefinitely less than that of our own at 45 miles from the earth. But we have 

 the clearest evidence that far within the limits of the corona the density must be almost infin- 

 itely less than that of our atmosphere at even CO miles above the earth's surface : for the great comet 

 of the last year passed at a distance of 300,000 miles of the sun, hence deep within the assumed 

 coronal atmosphere, for a space of several millions of miles and with a velocity 180 times that of 

 the earth in its orbit, not only without being stopped and precipitated upon the surface of the sun, 

 but without having been checked in the least. This was proved by the fact that the orbit deriv^ed 

 from observations after perihelion passage was sensibly the same as that before. Still more con- 

 clusive pi-oof is offered by the comet of 1843, since it passed still nearer the surface of the sun. It 

 is true that its oi'bit before and after perihelion passage does not admit of such comparison as iu 

 the more recent case, but we are forbidden by every law of probability, just because of its small 

 perihelion distance, to assume a higher velocity before nearest approach than that due to a para- 

 bolic orbit ; this it had after leaving the neighborhood of the sun. We must not, iu weighing this 

 evidence, overlook the fact that the resistance offered by an atmosphere increases with enormous 

 I'apidity with increasing velocity of motion. Surely, no more decisive argument against the exist- 

 ence of an atmosphere extending as far as the perihelion distance of either of these comets could 

 be imagined. 



But the assumption of an extensive atmosphere leads to a contradiction to our experience 

 wholly indepeudently of such considerations which would render it untenable. According to 

 theory, as well as observation, the upper limits of the gaseous envelopes of the sun ought to be 

 ordered according to their densities. The material which i)roduces the 1474 K line, and which 

 m.ay always be seen iu the chromosphere spectrum, is, according to this criterion, as unmistakably 

 denser than hydrogen as is magnesium vapor, or of iron vapor; but if we accept the coronal spec- 

 trum as evidence of the existence of an atmosphere, we are, bj' exactly the same principle, driven 

 to the conclusion that the 1474 K material is far less dense than hydrogen. The contradiction 

 could not be more abrui>t and inexiilicable. 



There are two other arguments of not inconsiderable weight which are opposed to the suppo- 



