and other Minerals when exposed to Heat. 33 



tensities, — 1, 2, 5, 4, 3, the areas following the natural order of the 

 numbers. Other series being taken with sets of crystals, 1.2 and 

 1 .8 inches long, gave similar indications of irregularity,* but the 

 area of section has so far a general influence, that where the dif- 

 ferences are considerable, the thickest crystal has almost univer- 

 sally the greatest power. The relative forces are so connected, 

 that we can hardly impute the irregularities to any general law ; 

 the differences, as I shall immediately illustrate by reference to 

 another class of experiments, must in all probability be attributed 

 to a variable structure in specimens of the same mineral, as well 

 as in those of different species. 



I took a crystal 1^ inches long, and carefully determined the 

 intensity of its electricity, which, by a mean of three experiments, 

 gave 45° of deviation. I immediately broke it at one-fourth of 

 its length from one end, the two portions being then heated and 

 their intensities determined each three times, the mean of the 

 larger portion gave a deviation of 47°, of the smaller 43°, the 

 mean of which gives precisely the original force. As far as in- 

 tensity goes, the diminution of length would not therefore ap- 

 pear to be favourable to the development of electricity. With 

 a view of procuring through a larger range of dimension, the in- 

 fluence of length alone, I selected a series of tourmalines whose 

 sections were as nearly equal as possible, the diameter being 

 about y-oth inch, and one of which was the very long crystal be- 

 fore mentioned. This experiment was made with great care ; a 



^03. 



Intensity. 

 1.2 long. 



Intensity. 

 1-8 long. 



1 (thickest) 



82° 



54° 



2 



77.5 



40° 



3 



50° 



34° 



4 



57.5 



35.5 



5 



65° 





6 (thinnest) 



34° 





VOL. XIII. PART I. E 



