32 Prof. Forbes's Experiments on the Electricity of Tourmaline 



use towards those of smaller dimensions. The intensity, too, 

 was very great, though more slowly attained than in shorter ones. 

 Various Tourmalines, between two and three inches in length, 

 uniformly shew great activity on being removed from the heat 

 to which they have been exposed, and left to cool, when applied 

 to the electroscope. 



This discovery led me to some inquiry into the effect of di- 

 mension in modifying electric action. Here it is necessary to 

 draw a distinction between the case of excitation, and the inten- 

 sity of the effect produced. M- Becquerel generally mentions 

 the temperature at which electricity appeared : my inquiries have 

 been directed to the maximum intensity of that electricity when 

 excited, which is in some respects the more satisfactory informa- 

 tion of the two. The determination of the temperature, we have 

 already seen, is a point of great uncertainty, since every range of 

 atoms, from the centre to the surface, must have a different tem- 

 perature. Of course, for the same reason, the maximum effect 

 is the integral of an infinity of variable forces. 



Amongst many experiments on different groups of crystals, I 

 may mention the following as the best determined. Six Tour- 

 malines, all 1.3 inches long, whose thicknesses or areas of section 

 were represented by the numbers 14, 11, 7, 6, and 4, had their 

 maximum intensities measured. Two series of experiments in a 

 direct order, and two reversed, all gave the same order of intensity 

 for these specimens, which, instead of bearing any direct propor- 

 tion to the areas, as might have been expected, where the lengths 

 were equal, gave the following arrangement* in the order of in- 



The best pair of series gave, 





No. 1, 



Deviation 115° 



2, 



69.5 



3, 







4, 



26 



5, 



395 



