we may suppose that the matter was in reality projected with a much greater velocity than 200 miles per second, and was brought to rest at a height of 200,000 miles by the retarding action of the solar atmosphere cooperating with solar gravity. And, of course, we may conceive that these two explanations coexist, and that the two causes considered operate with any degree of proportional activity, between the relations which would make one or other the sole cause of the observed excess of velocity.

Now, to determine the actual height which must be reached by a projectile from the sun (in vacuo) so that it may pass from a height of 100,000 to a height of 200,000 miles in ten minutes, I have gone through a series of calculations which need not be discussed here, leading to the result (which may be accepted as trustworthy) that 350,000 miles is the required height, and therefore 255 miles per second the requisite initial velocity. In this case the hydrogen wisps watched by Professor Young were in reality travelling at a rate of about 150 miles per second when they reached the highest visible part of their course and vanished from view as if by a process of dissolution.

On the other hand, it is not possible to determine the nature of the motion of hydrogen wisps, retarded by the resistance of the solar atmosphere, so as to travel from a height of 100,000 miles to an extreme height of 200,000 miles in ten minutes. We are very far from knowing how to deal satisfactorily with the motion of a solid projectile through our own atmosphere, which may be regarded as appreciably uniform during the projectile's flight, the action of terrestrial gravity being also appreciably uniform. But in the case of the solar atmosphere between the observed levels we have a problem infinitely more difficult, because the atmospheric pressure must be greatly less at a height of 200,000 miles than at a height of 100,000 miles, the solar gravity at these heights being also very different. Nor do we know what the atmospheric pressure is at either level. It would be mere waste of time to discuss a problem all the conditions of which are so vague.

But it will be worth while to consider the general relations which are involved.

In the first place, we may leave out of consideration the motion of the hydrogen before it reached the level of 100,000 miles. The retardation we have to enquire into is something taking place within the observed range of the projectile's motion, and we may consider the moving hydrogen precisely as though its motion had been due to some projectile force operating upon it when already at a height of 100,000 miles. Now we have seen that in order to traverse the next 100,000 miles above that level in ten minutes, it would require an initial rate

of motion (at that level) sufficient to carry it to a distance of 350,000 miles from the sun's surface if unretarded. But as the matter (on the hypothesis we are considering) did not reach this distance (250,000 miles from its starting place), but, on the contrary, only traversed a distance of 100,000 miles before being reduced to rest, it is obvious that its initial velocity (at level 100,000 miles) must have been greatly in excess of the velocity which, at that level, would correspond to an upward range of 350,000 miles in all. In other words, the hydrogen, when at a height of 100,000 miles, was travelling much faster than a projectile would cross that level if projected in vacuo at a rate of 255 miles per second. So that leaving out of consideration all the retardation experienced by the hydrogen before it reached the level 100,000 miles, its motion at that level corresponded to an initial velocity much exceeding 255 miles per second. But, if the retardation was so considerable between the levels 100,000 miles and 200,000 miles, as to reduce the hydrogen to rest at the last-named level, whereas in vacuo it would have reached a level much exceeding 350,000 miles, how much more effective must the retardation have been in the first 100,000 miles of the hydrogen's upward course? It is difficult to express how much greater must be the average density of the solar atmosphere between the photosphere and a height of 100,000 miles, than between the height 100,000 miles and 200,000 miles; but the disproportion must be enormous. Apart from this, the retardation being always proportioned to the velocity (though the law of this proportion is not known), would have been much more effective in the lower part of the hydrogen's course, on this account alone. We have, then, this important conclusion (on the hypothesis we are dealing with), that after traversing a range of 100,000 miles from the sun's surface under the action of a retardation enormously exceeding that operating on the hydrogen in the observed part of its flight, the uprushing hydrogen still retained a velocity far exceeding that due to a velocity of 255 miles per second at the sun's surface in the case of a projectile in vacuo.

But we have now to consider towards which hypothesis we should lean, or rather which cause we should consider as chiefly operative.

In the first place, it is obvious that we cannot dismiss the hypothesis of retardation entirely, for glowing hydrogen travelling through an atmosphere even of extreme tenuity at an average rate of 167 miles per second must needs be enormously retarded. But I think that, apart from this, we cannot for a moment accept the belief that the hydrogen wisps which Professor Young watched as they slowly vanished at a height of

200,000 miles were then travelling upwards at the rate of about 150 miles per second. So acute an observer could not but have recognised the fact that the hydrogen was still in rapid upward motion at that time. We are compelled then, as I judge, to regard retardation as operative to at least some considerable degree in that upper half of the hydrogen's

course.

This being so, I do not know that a single word of what I have said on the hypothesis of retardation being solely operative need be altered. The italicised words at the close of the remarks made on that view must still be used in stating the conclusion to which careful reasoning would lead us.

And here I approach the point to which these remarks have been tending. If we regard the hydrogen erupted or in motion in these jet prominences as not less dense than other matter partaking in the motion of primary ejection, the above conclusion, interesting as it is in itself, yet has no bearing on the subject of the corona. The erupted hydrogen reached a certain enormous altitude, and there (so far as the extrusion of matter from the sun was concerned), the work of the solar eruption came to an end. But we have seen that the spectrum of the jet prominences indicates the presence of several other elements amongst others, several metallic elements in the state of vapour. Now, it is highly probable that at a very early stage of the upward motion a large proportion of the metallic vapour would condense into the liquid form; and if so, such liquid metallic matter would thenceforward meet with far less resistance, and so would travel to a far greater distance than the hydrogen. But without insisting on this point, we may yet feel assured that under similar conditions of temperature and pressure the vapours of the metallic elements far exceed hydrogen in density. Thus they would from the very beginning of their upward course be exposed to a much less effective retarding influence. They would, therefore, retain a much greater proportion of the velocity primarily imparted to the whole body of erupted matter; nor is it by any means an unreasonable or unlikely supposition that at a height of 100,000 miles some of these constituents of the erupted matter would be travelling twice as rapidly upwards as the hydrogen watched by Professor Young. So far, indeed, is this view from being unlikely that it is difficult to entertain any other opinion. Yet, on this view, the matter referred to would be travelling at a rate greatly exceeding 400 miles per second; and a much smaller velocity would suffice to carry it away for ever from the sun's controlling influence. Much more, therefore, would the outrush of such matter suffice to explain the extension of the coronal streamers.

I shall merely note, in conclusion, that it would require only very moderate assumptions respecting the retarding influence of the solar atmosphere, to prove that the least of the jet prominences must have required a velocity of ejection competent to carry the vapours of metals as far as the outermost observed limits of the radiated corona. Now that we have such distinct and incontrovertible evidence of the retardation exerted above a height of 100,000 miles, the opinion* respecting the corona discussed by me in Fraser's Magazine' for last April, can no longer be regarded as other than a highly probable theory.

* It would be unfair not to mention, that this opinion had been much earlier urged, and very strong evidence in its favour adduced, by Mr. Matthew Williams, in his interesting work, "The Fuel of the Sun." It was still earlier suggested by the late Prof. Graham. I have been led to it, however, by a perfectly independent line of reasoning.

60

MADDER DYES FROM COAL.

BY EDWARD DIVERS, M.D., F.C.S.

HE beautiful dye-stuffs produced from coal-tar have all proved to be until recently such as were unknown bodies before they were obtained from this source. A new interest has now, however, been imparted to the colour-producing properties of coal-tar by the transformation of one of its constituents into one of the most important and beautiful vegetable dyes that are known-the dyeing matter derived from the madder (Rubia tinctorum). As the reader can hardly be assumed to be very familiar with what has been ascertained respecting the production of dyes from the madder-plant itself, it will be well to consider very briefly this production, which, though of some complexity, and still imperfectly comprehended, possesses considerable scientific as well as practical interest.

When powdered madder-root is spread upon an iron plate and cautiously heated so as to avoid scorching it, small orangecoloured crystals are seen to form upon its upper surface, which are the substance that imparts to the root its valuable dyeing power. This substance, discovered by Robiquet, is called alizarin, from alizari, the name by which madder is known in the Levant.

Alizarin, like indigo, appears, from the microscopic observations of Decaisne, not to exist ready formed in the juices of the living plant. The cells of the root are found to be filled with a yellow substance which increases in quantity with the age of the plant; and this substance gives rise to the true colouring matter when the juice is exposed to the air. Decaisne's observations were, however, disregarded by the chemists who investigated madder until Higgins drew attention to them again, and, by some very simple and admirably devised experiments, established their accuracy. From these experi

*In a paper read by him at the British Association in 1848. "Philosophical Magazine," XXXIII. 282.