1 34? Prof. Forbes on the Refraction aiid Polarization of Heat. 



The change of colour which is exhibited by looking through 

 the mineral in day- and in candle-W^^t^ arises from two causes : 

 1st, from there being an excess o^red and a defect of blue rays in 

 the light of a candle compared with the light of day ; and 

 2ndly, from the substance employed having a greater dispo- 

 sition to transmit one kind of rays in preference to another, or, 

 what is the same thing, being more transparent for one kind 

 of rays than for another kind, v/hen their intensity is the same. 



In the present mineral its colour \s green ; but when we ana- 

 lyse it with the prism we find that the green is a compound co- 

 lour consisting of red and green, the green predominating 

 greatly in dai/Vight : but in ca?idle\\g\it the colour is a piiik 

 red, because the greater quantity of red in this light and the 

 smaller quantity of blue and greeii, gives the red colour a de- 

 cided predominance over the green, so as to make the com- 

 pound colour pink red. 



There are several crystals, natural and artificial, and various 

 solutions in which this change of colour is beautifully seen. 

 It is particularly visible in the green ]mces of plants, which are 

 green in daylight, and of a blood red colour in the light of a 

 candle. 



In the mineral under our consideration Mr. Nordenskiold 

 found traces of the oxide of chromium, to which he attributes its 

 colour. That this is the colouring matter, and that the ac- 

 tion of this metal is the cause of its peculiar property in refer- 

 ence to light, may be inferred from the fact that the very same 

 property is possessed by the triple oxalate of chromium and 

 potash, and also by the sulphate of ammonia and chromium, 

 whether these salts are used in the solid state or in a state of 

 solution. 



XXII. On the Refraction and Polarization of Heat. By 

 James D. Forbes, Esq., F.R.SS. L. Sf E., Professor of Na- 

 tural Philosophy in the University of Edinburgh.* 



§ 1 . Some Miscella?ieous Experiments with the Thermo- 

 Multiplier. § 2. On the Polarization of Heat by 

 Tourmaline. § 3. On the Polarization of Heat by 

 Refraction and Reflection. § 4. On the Depolariza- 

 tion and Double Refraction of Heat. 



1. npHE experiments to be detailed in this paper, which 



-*■ chiefly go to establish properties of heat wholly un- 



looked for, or only suspected to exist, having been made en- 



* Communicated by the Author; having been read before the Royal 

 Society of Edinburgh on the 5th and 19th of January 1835. 



