Light, which Light is destroyed by the most luminous and the calorific rays; a fact which was previously noticed by Seebeck.*

(459.) M. Biot and the elder Becquerel have proved that the slightest electrical disturbance is sufficient to produce these phosphorescent effects. May we not, therefore, regard the action of the most refrangible rays as analogous to that of the electric disturbance? May not electricity itself be but a development of this mysterious solar emanation ?

(460.) A great many animals in the living state emit Light of the character which we have been considering:

the pholas, the medusa phosphorea, and many other mollusca; the lampyris or glow-worm, the fulgora, the scholopendra electrica, the cancer fulgens, and a variety of the annelids found in the bogs of Ireland, may be adduced as examples. Nearly all fish in a state of decomposition emit this kind of Light; and the flesh of most quadrupeds in the progress of putrefaction. Rotten wood, it is well known, also evolves a considerable quantity of Light in the dark. The dependence, or otherwise, of these phenomena upon solar influence, remains to be proved. In the Philosophical Transactions for 1790, Dr. Hulme published a very excellent Treatise on Phosphorescence, to which I must refer those who are desirous of obtaining further information on this part of the subject. I cannot refrain from suggesting the importance of experiments to determine if any class of the prismatic rays have the power of exciting or destroying the phosphorescence in living animals. It would appear from the experiments of Becquerel, that the luminous and calorific rays should possess that power. That artificial heat interferes with the phenomena in organised bodies is very certain. The whole question, however, is one of great difficulty, but it is an inquiry which is calculated to clear up much of the

*See Goethe's Optics.

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doubt which exists at present as to the theories of the emission of luminous particles, and the excitement of an all-pervading luminiferous ether.

Similar phenomena have been noticed in the vegetable world. The leaves of the anothera macrocarpa exhibit phosphoric light when the electrical intensity of the air is high. The agarics of the olive-grounds of Montpelier are often luminous at night. In the coal mines, near Dresden, the rhizomorpha phosphoreus shines with great brilliancy. Many of the lichens indeed are stated to possess this peculiar phosphorescent property; and from the circumstance of their existing in dark caverns, they appear to prove the conductibility of the luminous principle through masses of matter, or the power of production dependent upon vital force.

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CHAPTER IX.

INFLUENCE OF THE SOLAR RAYS ON CHEMICAL COMBINATION.

(461.) ALREADY, in the first part of this work, several examples, which show the influence of the sun's rays upon combination, have been brought forward: a few instances of a remarkable kind remain to be noticed. Vogel observed, that if chlorine was passed into alcohol nearly saturated with that gas, and at the same time exposed to the sunshine, each bubble of chlorine, as it entered the spirit, exploded, giving a bright purple flame and a white vapour. This experiment I have repeated, and found that the effect depends entirely upon the agency of the chemical radiations. The interposition of an orange glass, or a yellow fluid, is quite sufficient to stop this energetic chemical combination.

(462.) It has long been known to chemists, that a mixture of chlorine and hydrogen gases might be preserved in darkness, without combining, for some time; but that exposure to diffused daylight gradually occasioned their combination, whilst the direct solar rays produced the sudden inflammation of the mixture. This combination has been investigated by Gay Lussac, and Thenard, and also by Davy. Sir Humphry Davy states that a mixture of chlorine and hydrogen acted more rapidly upon each other, combining without explosion, when exposed to the red rays, than when placed in the violet rays. But he found that a solution of chlorine in water, became a solution of muriatic acid most rapidly, when placed in the most refrangible rays: the former statement is doubtful.

(463.) My own experiments appear to show that the combination of these gases may be effected in every part

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of the prismatic spectrum, but that it is entirely independent of the luminous rays. I have kept chlorine and hydrogen without uniting, behind a yellow medium, for as long a period as I have been able to preserve the mixture in the weakest diffused daylight. It does not, however, appear to be quite independent of calorific influence; for I find that the combination is effected gradually under the influence of the dark rays of heat.

(464.) We have evidence to show that the chemical agent, whatever it may be, which accompanies Light, is diffused over every part of the prismatic spectrum, although its action is modified by the luminous and calorific influences. Now, as it is proved that a very small amount of actinic power will occasion the chemical combination of these gases, we can well understand that it is diffused over the whole of the rays, although in different degrees. Dr. Draper has shown that the Light of a taper produces a decided effect upon the mixed gases, chlorine and hydrogen, and also that the Light emitted during the rapid passage of the electric spark, acts powerfully upon them. "For speed of action no tithonographic* compound can approach it; a Light which perhaps does not endure the millionth part of a second, affects it energetically." In the red ray the chemical influence is pretty active, and this, combined with the thermic power of that ray, accounts for the phenomenon observed by Davy. I have found, however, that the combination is effected with the greatest speed by the extreme blue and the indigo rays. Dr. Draper has fixed the maximum in the indigo rays, and giving a numerical value to the forces. exerted by the different rays, he calls the maximum power of the

* Tithonicity was a name given by Dr. Draper to the chemical rays; but which is, it appears to me, badly chosen; and certainly not at all in accordance with the Lavoiserian principle of nomenclature, which teaches, that the word should give birth to the idea, the idea depict the fact.

The red ray should have a much higher power than is here stated; I have found it quite equal to the green ray, and, I think, superior to it in effect. I should remark that, by using glass tubes of small bore, we secure the combination of the gases without any explosion.

(465.) Taking advantage of the action of the sun's rays upon these gases, Dr. Draper devised an instrument for measuring the chemical force exerted by Light. This instrument consists essentially of a mixture of equal volumes of chlorine and hydrogen, which is evolved from, and confined over muriatic acid, in a graduated bent tube. The gases are liberated from the liquid acid by the agency of galvanic electricity. Platinum wires, which can be connected with a voltaic battery, are inserted into the tube in such a manner, that when the required quantity of the gases is formed, the decomposition ceases, owing to the fluid having fallen below the wires. The gases combine in a longer or shorter time, according to the amount of Light; the number of degrees over which the fluid falls in the graduated arm in a minute giving relatively the force in action. This instrument is certainly a very ingenious application. But it appears to me, there are so many causes which will operate to produce an irregular action, that the results obtained by such an instrument can only be received as approximations to the truth, and indeed not that, unless the average of a great many carefully conducted experiments be taken in every case.

(466.) The formation by the sun's rays of precipitates which do not occur in the dark, has engaged the attention of Sir John Herschel; but further investigations