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



their relations must be considered extremely important. I have 

 therefore one remark to make upon an experiment which Dr 

 Brewster thinks indicative of a " singular breach of analogy 

 between the distribution of the pyro-electrical and magnetical 

 forces." After observing that, in the process of reducing a mag- 

 net to powder, the coercive force employed effectually destroys 

 all trace of magnetism, he adds that powder of tourmaline is 

 highly electric when placed on a glass and heated, which is shewn 

 by its adhering in conglomerated masses, exhibiting the appear- 

 ance of viscidity when stirred. It appears to me that this expe- 

 riment does not go to shew that tourmaline in a state of excita- 

 tion does not lose its electricity when bruised in a mortar ; in- 

 deed, such an experiment it would be impossible to perform. A 

 tourmaline, when it is not changing its temperature, is as inert 

 as a bar of iron before it is magnetized ; the process of heating 

 or cooling the one, is precisely equivalent' to that of conveying 

 magnetism by induction or otherwise to the other. The powder 

 of tourmaline is, therefore, analogous to the filings of iron, both 

 being equally inert, till the native electricity of the former, and 

 the native magnetism of the latter, is decomposed, when the re- 

 sult in both is perfectly identical. 



I shall now only very briefly allude to the conclusions to 

 which some experiments on the electricity of other minerals be- 

 sides tourmaline have led me. I have applied Coulomb's elec- 

 trometer with perfect success to the examination of topaz, bora- 

 cite, and mesotype, which have all been long known to possess 

 electrical powers. In the case of these minerals, I have been 

 able to extend Becquerel's remarkable law of the intensity of 

 electricity rising to a maximum, when the speed of cooling has 

 become comparatively low, which has not before been demon- 

 strated for any mineral except tourmaline. Topaz possesses the 

 remarkable property of retaining its electricity long after the 

 temperature has ceased to change : probably the decomposition 



