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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.
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 CHLORINE AND HYDROGEN.
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
CHLORINE, IODINE, AND BROMINE.
are required. A few examples will be found in a future paragraph.
(467.) Phenomena which I have observed lead me to believe that under no circumstances, where the changes are gradual, does precisely the same thing take place in darkness as in daylight.
As far as my own observations have gone, I find that in all cases where precipitation does not take place immediately upon mixing two solutions, there is a very marked difference in the time required for precipitation to ensue in a fluid kept in the dark, and one exposed even to diffused daylight, this being, of course, more strikingly shown if one fluid is placed in the sunshine. Some interesting experiments illustrating this part of the subject will presently be given.
(468.) Chlorine, iodine, and bromine, it is well known, act with considerable energy upon metallic bodies. If, however, any polished metal is exposed to the action of them in a diluted state, the combination is, at first, exceedingly weak, and the films that are formed by either of these three elementary bodies, upon any metal, undergo considerable change under the influence of the sun. In most cases it appears that these bodies are set free, and the metal left in a state of very fine division or oxidisation. Copper, tin, iron, zinc, lead, pewter, bismuth, and several other metals have afforded the same results. It is still more remarkable, that films of bromine or iodine on glass are found, under the action of the sun, to act in a similar manner; and in 1841 I published an account of the power of iodine in rendering wood capable of receiving photographic images.
(469) In connexion with this section of my subject, the following observations and experiments of Dr. Frankland are most important. They are abstracted from his " Researches on the Organic Radicals,” published in the Quarterly Journal of the Chemical Society. "Scheele, Seebeck, and others, found that nitric acid