206 Prof. Forbes 07i the Refraction and Polarization of Heat. 



dergo any similar change in similar circumstances. I made a 

 preliminary experiment with heat from an oil-lamp (not an 

 Argand), and though, when the axes were crossed, the whole 

 light was stopped^ the heat transmitted appeared to be as in- 

 tense as before. The tourmalines which I employed were 

 mounted on glass, and were kindly lent to me by the Rev. 

 Mr. Craig. Struck with the singularity of the result, I re- 

 peated the experiment with additional precautions, and I found 

 that some circumstances prevented this statement from being 

 true in all its generality. The quantity of heat transmitted 

 being very small, the lamp, the tourmalines, and the pile were 

 very near to one another; and, as the tourmaline absorbs 

 heat with great rapidity, I found that a minute difference 

 might exist if the experiment was made first with the axes 

 parallel, and then with the axes crossed, which difference 

 might yet be made up by the secondary radiation from the 

 heated tourmaline, which was constantly becoming more in- 

 tense. Such at least appeared to be the chief source of error, 

 which I am particular in stating, because I afterwards dis- 

 covered that M. Melloni had been led to the very same con- 

 clusion as I at first was, and had published it. 



20. When I proceeded to verify my results by a series of 

 successive observations, under the two conditions of axes pa- 

 rallel and axes crossed, so as to eliminate any error from a 

 constantly progressive change, I perceived my mistake. As 

 this illustrates the method by which almost all my observa- 

 tions have been reduced, I shall give an example. Two mea- 

 sures of intensity in the position where least light was trans- 

 mitted, which is marked Dark, have their mean taken, which 

 is then compared with the intervening observation in the po- 

 sition of greatest illumination, which is marked Light, These 

 tourmalines we may call A and B. 



1834-, Dec. 4. — Oil Lamp* six inches from Centre of the Pile, 



Deviations of 

 galvanometer. 



♦ The oil lamp used when not expressly called « Argand," was Locatelli's 

 lamp with a solid square wick, which is what M. Melloni employed. 



