36 Prof. Forses’s Laperiments 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 Coutoms’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 Brecqurre.’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 
