Popular Science Monthly/Volume 2/December 1872/Professor Tyndall's Topics

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THAT the expectation of pleasure and profit from the course of lectures which Prof. Tyndall has prepared for delivery in this country is not likely to be disappointed, will appear from the following careful analysis of his theme, Light and Heat, as he has arranged it for six nights:

He begins in a prefatory way, and dwells upon the introduction of the experimental method into Science—speaks of the ardor of investigators and of their rewards. He seeks to show that most of them wrought for the sake of knowledge, and with no practical end in view, though their discoveries travelled to the most astonishing practical applications. After dwelling on the importance of original inquiry, he takes up the real subject of the lectures. The instruments are explained, and the principles upon which they depend. He points out the proximate cause and action of the electric light which is to be used in the illustrations. The laws of reflection are demonstrated, and one or two striking practical applications adduced. Then he goes on to refraction. These elementary subjects are really touched upon in order to enable him, in a subsequent lecture, to reveal the workings of Newton's mind when he theorized upon the subject of light. Refraction is followed by an inquiry into the constitution of light, its analysis and synthesis. This occupies the first lecture.

In the second lecture the demonstrated constitution of light is applied to the doctrine of colors. He goes very thoroughly and plainly into this matter, making perfectly evident the causes on which ordinary colors depend; winding up by the experimental proof that yellow and blue light, when mixed together, produce white and not green. Having exhausted the ordinary spectrum, he describes the difference between the emissions from solids and their vapors. Metallic vapors are produced and shown with their characteristic colors. Their light is then analyzed, and it is shown to be distinctive of the substance from which it comes. Spectrum analysis is dwelt upon, and copiously illustrated.

In his third lecture, Tyndall deals with solar light, dwelling upon the distinction between the bright lines of the metallic vapors, and the dark lines of Frauenhofer. The reciprocity of radiation and absorption is demonstrated, and it is shown experimentally that an incandescent vapor absorbs the light which it emits. This leads up to the theory of the physical constitution of the sun. Then he goes on to show the extension of the spectrum beyond its visible range, performing with quartz prisms and lenses Stokes's experiments on Fluorescence, and the rendering visible of invisible rays. Then the other side of the spectrum is handled; its extension as heat beyond the limits of light is demonstrated. Numerous experiments on the total heating power of the rays from the electric light are made; fusion and combustion being thus effected. Here he hopes to perform the famous Florentine experiment of the ignition of a diamond in oxygen, using, however, a purely terrestrial source of radiant heat. He also hopes to produce combustion by rays concentrated by a lens of ice. The heat-rays are then filtered from the light-rays, and it is shown that all the foregoing effects are produced by rays totally beyond the range of vision; fusion, combustion, and explosion, being produced at foci perfectly dark, and, as far as the air is concerned, perfectly cold. It is also proved that these dark rays perform the work of evaporation in the tropical ocean, and the work of fusion upon the Alpine ice and snows. The rays, moreover, are shown to be competent to raise platinum to a white heat, so that by its intervention you may extract from the dark rays all the colors of the spectrum. This brings him to the end of the third lecture.

In the fourth lecture he shows the irresistible tendency of the human mind to seek for governing principles which rule facts and correct them, rendering them, so to say, organic. He dwells upon the exercise of the theorizing faculty, taking Newton as an example. He tries to show how naturally his optical theory grew out of his previous knowledge. The doctrine of colors is now extended by the introduction of the colors of thin plates, of striated surfaces, etc., and he unravels the subtle additions which Newton made in his theory, in order to fit it to these new facts. The theory of emission is then contrasted with the theory of undulation. The latter is rendered familiar to the mind by preliminary considerations regarding water-waves, and by experiments regarding sound. He dwells upon the labors of Thomas Young, and the effect of Brougham's attacks in the Edinburgh Review. This will be his most difficult lecture, but he has wrought hard to make it clear, and it is essential to the comprehension of the subsequent ones.

In the fifth lecture Tyndall enters upon the phenomenon of crystallization, and seeks to give an intelligible explanation of crystalline architecture. The process of crystallization is experimentally illustrated. This is done with a view to the action of crystals upon light. In the first experiments he deals with crystals solely with reference to the polarization of light. This is explained and illustrated by numerous experiments. Double refraction and the state of the two halves of the divided beam are dwelt upon. Then come the chromatic phenomena of polarized light. Basing himself upon the principles explained in the fourth lecture, he hopes to make these effects comprehensible by all intelligent persons. The effects of mechanical strains and pressures in producing a quasi crystalline structure are exhibited. Then the similar phenomena of unannealed glass. He hopes to show these effects in a very splendid fashion. They will more than fill the fifth lecture.

The sixth lecture is devoted to the further illustration of the action of crystals upon light; uniaxal and biaxal crystals, circular polarization, and the chromatic effects produced by rock-crystals; the conferring of double refractory power by sonorous vibrations; and the magnetization of light. Although the syllabus is short, it covers a good deal.

We have sketched the course of six lectures. The materials touched upon are ample to fill the six to overflowing, allowing an hour and a half for each lecture. A seventh very striking lecture might be given, he says, on the identity of light and heat—every experiment made in the optical lecture being shown capable of repetition with pure lightless radiant heat, the thermo-electric pile and galvanometer being substituted for the eye. He has made an arrangement for the projection of the galvometer-dial upon a screen, which renders it visible to any number of people.

As he worked at the subject, the desire grew upon him to do it more and more thoroughly, and to spare no expense as regards apparatus. He has accordingly purchased between three and four hundred pounds sterling worth of new instruments; and has gone over all the experiments, so as to render every thing sure, and in a manner worthy of the subject and of the occasion.—United States Railroad and Mining Register.