340 Dr. Seebeck on the unequal Evolution 



Exp. 35. (7th September 1806.) A prism filled with water, 

 whose refracting angle was about 40'\ The panes of glass 

 which inclosed the water had a breadth of about four inches. 

 The prism was brought as near as possible to the normal posi- 

 tion, and the spectrum received at the distance of about five feet. 

 The same water-prism was applied in Experiment 36. The 

 thermometer at the distance of four feet. In both experiments 

 the colourless light in the centre of the spectrum always pos- 

 sessed the greatest degree of heat ; the next in degree wtis the 

 light above the yellow. In the red, however, the temperature 

 was much lower; the difference amounting to an inch. It is 

 also remarkable, that the temperature in the white border un- 

 der the blue approached that in the red, it being but three lines 

 higher. I must not omit to state, that a second and weaker 

 yellow and red (as first observed by Ritter in glass prisms) 

 appeared also in this prism, immediately below the blue. 



Two other experiments (37 and 38) were made with this 

 water-prism, at distances from the thermometer of from four 

 to five feet. Both experiments evinced that close under the 

 red the heat is much less than in the red or in the yellow. And 

 Experiment 39, made at the distance of six feet, not only con- 

 firmed the preceding experiments, but proved also that the 

 heat in the yellow exceeds that in the red*, and this at the 

 distance mentioned, (yet all other conditions being balanced,) 

 by one inch. 



Exp. 40. — The prism was filled with a solution of sal am- 

 moniac and corrosive sublimate, a mixture to which Blair, in 

 his treatise on aplanatic telescopesf , ascribes a particular power 

 of dispersing colours, and whose application therefore, for the 

 purpose of a comparison with the effect of water, might be 

 found useful. This solution was quite colourless and trans- 

 parent, the dispersion of colour very considerable, and the 

 great extent of the red colour particularly striking. Distance 

 of the thermoscope from the prism four feet and a half. 



The highest temperature in this case was also in the white, 

 and below the red a less degree of heat than in the I'ed ; and 

 according to two other similar experiments (41 and 42) the 

 place of the greatest heat with this refracting medium seems 

 to be between the yellow and the red, but nearer the former 

 than the latter. Even when the equilateral prism used in 

 Experiment 42 was filled with colourless concentrated sul- 



• The same result was obtained previously by Wiinsch. See Mag. 

 d. Gesellschaft naturforsckendcr Freunde in Berlin, 1807, 1 Jakrg. 4 ke/t. 

 He also found the greatest heat in the yellow with prisms filled with spirits 

 of wine and oil of tiu-pcntine. 



t Gi/b. An. d, Phys. vol. vi. p. 129. 



phuric 



