140 Prof. Forbes's Experiments on the Electricity of 



The mean deviations of the needle of the electroscope will be 

 given in the following Table. 



No. Length. Intensity. 



1 325 inch 79°*5 



2 2-10 82 



3 1*60 60 



4 1-5.5 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 re- 

 presents best the pha?nomena of the tourmaline, is that of a 

 series of insulated plates of glass arranged parallel to one an- 

 other, suitably coated, and with the contiguous coatings con- 

 nected by tinfoil. 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 communicated, whilst those at the other will 

 have the opposite species 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 nu- 

 merous), 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 tour- 

 maline; but when we come to consider 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 consi- 

 derable area, and so formed as to have a large surface, per- 

 haps the most energetic. 



The unequal temperature of the portion of any section pre- 

 vents, as I have already observed, all the parts from giving a 

 maximum intensity at once. This will diminish the total effect, 

 but as all the parts afford the same kind of electricity, the re- 

 sultant can never be null on this account. Therefore even if 

 the irregularities of amount did not compel us to admit innate 

 varieties of structure, or electric disposition in different spe- 

 cimens, stubborn facts must force us to some such conclusion. 

 In the course of my researches I have met with a crystal of 

 tourmaline * possessing no external irregularities of structure, 



* It is No. 3. of the Series at the foot of p. 139. 



