New Chemical Reactions Produced By Light
( Originally Published 1905 )
MEASURED by their power, not to excite vision, but to produce heat—in other words, measured by their absolute energy—the ultra-red waves of the sun and of the electric light, as shown in the preceding articles, far transcend the visible. In the domain of chemistry, however, there are numerous cases in which the more powerful waves are ineffectual, while the more minute waves, through what may be called their timeliness of application, are able to produce great effects. A series of these, of a novel and beautiful character, discovered in 1868, and further illustrated in subsequent years, may be exhibited by subjecting the vapors of volatile liquids to the action of concentrated sunlight, or to the concentrated beam of the electric light. Their investigation led up to the discourse on "Dust and Disease," which follows in this volume; and for this reason some account of them is introduced here.
A glass tube three feet long and three inches wide, which had been frequently employed in my researches on radiant heat, was supported horizontally on two stands. At one end of the tube was placed an electric lamp, the height and position of both being so arranged that the axis of the tube, and that of the beam issuing from the lamp, were coincident. In the first experiments the two ends of the tube were closed by plates of rock-salt, and -subsequently by plates of glass. For the sake of distinction, I call this tube the experimental tube. It was connected with an air-pump, and also with a series of drying and other tubes used for the purification of the air.
A number of test-tubes, like F, Fig. 2 (I have used at least fifty of them), were converted into Woulf's flasks.
Each of them was stopped by a cork, through which passed two glass tubes : one of these tubes (a) ended immediately below the cork, while the other (b) descended to the bottom of the flask, being drawn out at its lower end to an orifice about 0.03 of an inch in diameter. It was found necessary to coat the cork carefully with cement. In the later experiments corks of vulcanized India-rubber were invariably employed.
The little flask, thus formed, being partially filled with the liquid whose vapor was to be examined, was introduced into the path of the purified current of air. The experimental tube being exhausted, and the cock which cut oft the supply of purified air being cautiously turned on, the air entered the flask through the tube b, and escaped by the small orifice at the lower end of b into the liquid. Through this it bubbled, loading itself with vapor, after which the mixed air and vapor, passing from the flask by the tube a, entered the experimental tube, where they were subjected to the action of light.
The whole arrangement is shown in Fig. 3, where L rep-resents the electric lamp, s s' the experimental tube, p p' the pipe leading to the air-pump, and F the test-tube containing the volatile liquid. The tube t t' is plugged with co ton-wool intended to intercept the floating matter of the air; the bent tube T' contains caustic potash, the tube T sulphuric acid, the one intended to remove the carbonic acid and the other the aqueous vapor of the air.
The power of the electric beam to reveal the existence of anything within the experimental tube, or the impurities of the tube itself, is extraordinary. When the experiment is made in a darkened room, a tube which in ordinary daylight appears absolutely clean is often shown by the present mode of examination to be exceedingly filthy.
The following are some of the results obtained with this arrangement:
Nitrite of Amyl.—The vapor of this liquid was, in the first instance, permitted to enter the experimental tube while the beam from the electric *lamp was passing through it. Curious clouds, the cause of which was then unknown, were observed to form near the place of entry, being afterward whirled through the tube.
The tube being again exhausted, the mixed air and vapor were allowed to enter it in the dark. The slightly convergent beam of the electric light was then sent through the mixture. For a moment the tube was optically empty, nothing whatever being seen within it; but before a second had elapsed a shower of particles was precipitated on the beam. The cloud thus generated became denser as the light continued to act, showing at some places vivid iridescence.
The lens of the electric lamp was now placed so as to form within the tube a strongly convergent cone of rays. The tube was cleansed and again filled in darkness. When the light was sent through it, the precipitation upon the beam was so rapid and intense that the cone, which a moment before was invisible, flashed suddenly forth like a solid luminous spear. The effect was the same when the air and vapor were allowed to enter the tube in diffuse daylight. The cloud, however, which shone with such extraordinary radiance under the electric beam, was invisible in the ordinary light of the laboratory.
The quantity of mixed air and vapor within the experimental tube could, of course, be regulated at pleasure. The rapidity of the action diminished with the attenuation of the vapor. When, for example, the mercurial column associated with the experimental tube was depressed only five inches, the action was not nearly so rapid as when the tube was full. In such cases, however, it was exceedingly interesting to observe, after some seconds of waiting, a thin streamer of delicate bluish-white cloud slowly forming along the axis of the tube, and finally swelling so as to fill it.
When dry oxygen was employed to carry in the vapor, the effect was the same as that obtained with air.
When dry hydrogen was used as a vehicle, the effect was also the same.
The effect, therefore, is not due to any interaction between the vapor of the nitrite and its vehicle.
This was further demonstrated by the deportment of the vapor itself. When it was permitted to enter the experimental tube unmixed with air or any other gas, the effect was substantially the same. fence the seat of the observed action is the vapor.
This action is not to be ascribed to heat. As regards the glass of the experimental tube, and the air within the tube, the beam employed in these experiments was perfectly cold. It had been sifted by passing it through a solution of alum and through the thick double-convex lens of the lamp. When the unsifted beam of the lamp was employed, the effect was still the same; the obscure calorific rays did not appear to interfere with the result.
My object here being simply to point out to chemists a method of experiment which reveals a new and beautiful series of reactions, I left to them the examination of the products of decomposition. The group of atoms forming the molecule of nitrite of amyl is obviously shaken asunder by certain specific waves of the electric beam, nitric oxide and other products; of which the nitrate of amyl is probably one, being the result of the decomposition. The brown fumes of nitrous acid were seen mingling with the cloud within the experimental tube. The nitrate of amyl, being less volatile than the nitrite, and not being able to maintain itself in the condition of vapor, would be precipitated as a visible cloud along the track of the beam.
In the anterior portions of the tube a powerful sifting of the beam by the vapor occurs, which diminishes the chemical action in the posterior portions. In some experiments the precipitated cloud only extended half-way down the tube. When, under these circumstances, the lamp was shifted so as to send the beam through the other end of the tube, copious precipitation occurred there also.
Solar light also effects the decomposition of the nitriteof-amyl vapor. On October 10, 1868, I partially darkened a small room in the Royal Institution, into which the sun shone, permitting the light to enter through an open portion of the window-shutter. In the track of the beam was placed a large plano-convex lens, which formed a fine convergent cone in the dust of the room behind it. The experimental tube was filled in the laboratory, covered with a black cloth, and carried into the partially darkened room. On thrusting one end of the tube into the. cone of rays behind the lens, precipitation within the cone was copious and immediate. The vapor at the distant end of the tube was in part shielded by that in front, and was also more feebly acted on through the divergence of the rays. On reversing the tube, a second and similar cone was precipitated.