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.
