PROFESSOR TTNDALVS TOPICS. 213 



demonstrated. Numerous experiments on the total heating power of 

 the rays from the electric light are made ; fusion and combustion "be- 

 ing 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 ; fu- 

 sion, combustion, and explosion, being produced at foci perfectly dark, 

 and, as far as the air is concerned, pei-fectly 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 hu- 

 man 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 knowl- 

 edge. 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 crystal- 

 lization, and seeks to give an intelligible explanation of crystalline 

 architecture. The process of crystallization is experimentally illus- 

 trated. 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 numer- 

 ous experiments. Double refraction and the state of the two halves 

 of the divided beam are dwelt upon. Then come the chromatic phe- 

 nomena of polarized light. Basing himself upon the principles ex- 

 plained in the fourth lecture, he hopes to make these effects compre- 

 hensible 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. 



