of Heat in the Prismatic Spectrum. 339 



refraction and dispersion of colours, the maximum of heat fell 

 within the red, whilst in flint-glass it always fell below it. T!ie 

 prism No. 6 seems to have the maximum of heat on the 

 boundary of the red. 



I cannot omit mentioning here a result of another kind, 

 which goes to confinn the different effect of the species of 

 glass just named. White muriate of silver, as I have detailed 

 elsewhere *, is changed in the spectra given by prisms of com- 

 mon white glass (among the rest in that of the prism No. 1) 

 in the following manner. In the violet it turns to a reddish- 

 brown colour, and even beyond the boundaries of that ray: in 

 the blue it turns blue, or blueish-gray ; in the yellow it re- 

 mains nearly unchanged, or at most assumes a slight yel- 

 lowish tinge ; but in the red, and mostly also a little beyond 

 it, it turns red itself In some prisms (I have also said) this 

 redness fell entirely without the red of the spectrum, — they 

 were such with which the greatest warmth was produced be- 

 yond the red ; and I may add here, that it was with the prisms 

 Nos. 2 and 3 that I made this experiment. 



I undertook some further experiments with the prism No. 6 

 (Experiments 24 — 28); and another prism, No. 7, in Experi- 

 ment 27, was pretty equal in effect with prism No.6 ; the high- 

 est point of heat appearing also with this close to the limit of 

 the red, and perhaps a little beyond it. An eighth prism, ot the 

 colour of the smoky topaz, proved exactly the same; the greatest 

 warmth being for the most part on the limit of the red, al- 

 though the temperature was sometimes found equal in the 

 red itself and immediately below it. The effect of another 

 prism. No. 9, of a yellowish tinge, with which the Experiments 

 29 and 30 were made, approached that of the three former. 

 Its considerable weight and strong refraction and dispersion 

 of colours make it probable that it contains lead, which may 

 also have given it its yellow colour. A prism of German 

 flint-glass, on the other hand, gave exactly the same result 

 (Experiment 31) as those of the prisms No's. 2 and 3 of En- 

 glish flint-glass; the greatest heat was observed when the 

 bulb of the thermoscope stood three lines below the limit (;f 

 the red. — But three other prisms (No. 10 of common glass, 

 No. 11 of white Bohemian, and No. 12 of white glass) had 

 the same results (in Experiments 32 — 34) as Nos. 1, 4, and 5 : 

 they showed the greatest heat when the bulb of the thermo- 

 meter stood in full red. 



I made also several experiments with liquids, of which I shall 

 select a few. 



* Goethe's Farlienlclire, vol. ii. p. 718. 



U u 2 Exp. 3.'5- 



