SECTION VIII.

THEORY OF THE ACTINIC ACTION OF THE SOLAR RAYS.

Experiments and hypotheses of Tyndall-Originality of his theory; objections and omissions-Is this theory incompatible with that of a repulsive force?

A NEW theory of cometary phenomena which has been proposed by Professor Tyndall, one of the most distinguished of contemporary physicists, in our opinion merits special attention. In the first place, because we believe it to be altogether new and original; and, in the second place, because it is derived, not from à priori conceptions, like so many other theories in astronomy and physics, but from accurate experiments and their interpretation.

The study of the action of radiations upon very rarefied media of gaseous matter first led Professor Tyndall to consider the mode of production of the phenomena presented by the heads and tails of comets. Of the undulations proceeding from any luminous source, such as the sun, some have a purely calorific action; these are those which have the greatest amplitude or are least refrangible; the undulations which constitute or produce light come next in the order of length of wave or refrangibility; the shortest waves are those which manifest themselves exclusively by chemical action. We now proceed to explain Professor Tyndall's views on the subject of these modifications, and his manner of accounting for the fact that the

rays of shortest wave-length are endowed with the property of acting upon chemical substances, decomposing them and separating the atoms of which their molecules are composed, whilst the larger and mechanically more powerful waves are, on the contrary, ineffectual to perform any such decomposition.

'Whence, then, the power of these smaller waves to unlock the bond of chemical union? If it be not a result of their strength, it must be, as in the case of vision, a result of their periods of recurrence. But how are we to figure this action? I should say thus: the shock of a single wave produces no more than an infinitesimal effect upon an atom or a molecule. To produce a larger effect the motion must accumulate; and for wave-impulses to accumulate they must arrive in periods identical with the periods of vibration of the atoms on which they impinge. In this case each successive wave finds the atom in a position which enables that wave to add its shock to the sum of the shocks of its predecessors. The effect is mechanically the same as that due to the timed impulses of a boy upon a swing. The single tick of a clock has no appreciable effect upon the unvibrating and equally long pendulum of a distant clock; but a succession of ticks, each of which adds, at the proper moment, its infinitesimal push to the sum of the pushes preceding it, will, as a matter of fact, set the second clock going.'

After having thus explained the chemical action of light, Professor Tyndall proceeds to study its action upon the vapours of different volatile substances, sometimes employing a beam of electric light, and at other times the solar light. He fills a tube of certain length with a mixture of air and the vapour of nitrite of amyl, of nitrate of butyle, or of iodide of allyl, after having taken the requisite precautions for the exclusion of all foreign matters, and more especially of particles floating in the air-dust, organic germs, mineral matters, &c. When thus

filled the tube remains dark, and the mixture it contains is absolutely invisible. But should a luminous beam of light, such as that given by the flame of a lamp, be rendered convergent by a lens and allowed to fall upon the interior of the tube, the following will be observed: the space for an instant after the introduction of the beam will remain dark; but this brief moment passed, a white luminous cloud will be seen to invade that portion of the tube occupied by the beam of light. How has this change been effected? The action of the waves has decomposed the nitrite of amyl and precipitated a rain of particles which from that moment are capable of reflecting and diffusing in all directions the light of the beam. 'This experiment,' says Tyndall, 'illustrates the fact, that however intense a beam of light may be, it remains invisible until it has something to shine upon. Space, although traversed by the rays from all suns and all stars, is itself unseen. Not even the ether which fills space, and whose motions are the light of the universe, is itself visible.'

We may see by this last remark the capital objection which forces astronomers to reject the theory of Cardan, according to which the tails of comets are simply the effect of refraction. This theory we have already mentioned.

It is to be remarked that the end of the experimental tube most distant from the lamp is free from cloud. Now, the nitrite of amyl vapour is there also, but it is unaffected by the powerful beam passing through it. Why? Because the very small portion of the beam competent to decompose the vapour is quite exhausted by its work in the frontal portions of the tube; it is the longer waves that continue their course; but these waves are powerless to produce a chemical decomposition. Thus can the able physicist find in the detail of facts the confirmation of his ingenious hypotheses. But let us now proceed to the theory of cometary phenomena.

The substance of this theory has been embodied by Professor Tyndall in the seven following propositions, which we will reproduce in the author's own words:

1. The theory is, that a comet is composed of vapour decomposable by the solar light, the visible head and tail being an actinic cloud resulting from such decomposition; the texture of actinic clouds is demonstrably that of a comet.

2. The tail, according to this theory, is not projected matter, but matter precipitated on the solar beams traversing the cometary atmosphere. It can be proved by experiment that this precipitation may occur either with comparative slowness along the beam, or that it may be practically momentary throughout the entire length of the beam. The amazing rapidity of the development of the tail would be thus accounted for without invoking the incredible motion of translation hitherto assumed.

3. As the comet wheels round its perihelion, the tail is not composed throughout of the same matter, but of new matter precipitated on the solar beams, which cross the cometary atmosphere in new directions. The enormous whirling of the tail is thus accounted for without invoking a motion of translation.

4. The tail is always turned from the sun, for this reason: two antagonistic powers are brought to bear upon the cometary vapour-the one an actinic power, tending to effect precipitation; the other a calorific power, tending to effect vaporisation. Where the former prevails, we have the cometary cloud; where the latter prevails, we have the transparent cometary vapour. As a matter of fact, the sun emits the two agents here invoked. There is nothing whatever hypothetical in the assumption of their existence. That precipitation should occur behind the head of the comet, or in the space occupied by the head's shadow, it is only necessary to

assume that the sun's calorific rays are absorbed more copiously by the head and nucleus than the actinic rays. This augments the relative superiority of the actinic rays behind the head and nucleus, and enables them to bring down the cloud which constitutes the comet's tail.

5. The old tail, as it ceases to be screened by the nucleus, is dissipated by the solar heat; but its dissipation is not instantaneous. The tail leans towards that portion of space last quitted by the comet-a general fact of observation being thus accounted for.

'6. In the struggle for mastery of the two classes of rays a temporary advantage, owing to variations of density or some other cause, may be gained by the actinic rays, even in parts of the cometary atmosphere which are unscreened by the nucleus. Occasional lateral streamers, and the apparent emission of feeble tails towards the sun, would be thus accounted for.

'7. The shrinking of the head in the vicinity of the sun is caused by the breaking against it of the calorific waves, which dissipate its attenuated fringe and cause its apparent con

traction.'

This very brief exposition of an hypothesis which might be termed the physico-chemical theory is taken from the new edition of Tyndall's work upon Heat. It is unaccompanied by explanation or commentary of any kind, and to us at least seems to be wanting in completeness, and to contain some obscurities. which we shall briefly notice, in the form of questions and objections, and on the subject of which we should be glad to receive additional elucidation from the author.

Professor Tyndall defines comets without making mention of the nucleus. Comets, for him, would appear to be simple masses of vapour rendered visible by the actinic action of the solar rays. Further on, nevertheless, he considers the nucleus