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



direct and reversed series were taken, and several of the deter- 

 minations independently repeated. The mean deviations of the 

 needle of the electroscope will be given in the following Table : 



No. 



Length. 



Intensity. 



1 



3.25 inch. 



79.°5 



2 



2.10 



82° 



3 



1.60 



60° 



4 



1.55 



60° 



5 



1.35 



89° 



6 



1.19 



68° 



We thus see that the long crystal holds a high place among 

 those of equal section with it, and we have at the same time an 

 additional proof of the native irregularities of different crystals. 



It is well known that the artificial arrangement which repre- 

 sents best the phenomena of the tourmaline, is that of a series of 

 insulated plates of glass arranged parallel to one another, suit- 

 ably coated, and with the contiguous coatings connected by tin- 

 foil. If one end of this battery be charged from an electrical 

 source, while the other communicates with the ground, the plates 

 at one extremity will partake of an excess of the electricity com- 

 municated, whilst those at the other will have the opposite spe- 

 cies in excess, and a large proportion of the range in the centre 

 will exhibit no traces of free electricity: hence, by shortening the 

 pile (supposing the plates very numerous), no change will take 

 place in the intensity of the free electricity, but the intensity will 

 bear a direct relation to the surface of the plates, or the section 

 of the pile. So far analogy supports the increase of intensity 

 with the diameter of the tourmaline ; but when we come to con- 

 sider the mode of charging, it fails, and leaves us in great doubt 

 as to whether the length of a crystal, if its structure be perfectly 

 uniform, should have any influence or not. I have found short 

 crystals of a considerable area, and so formed as to have a large 

 surface, perhaps the most energetic. 



