Dr. Brewster on the laws of polarisation, &c. 253 
’entirely without some general indications. In magnetism 
and electricity, the various phenomena are produced by two 
opposite and co-existent forces which modify each other's 
action ; and since opposite forces are obviously indicated by 
the phenomena of polarisation, we have the strongest reasons, 
from analogy, to believe that they are also co-existent in 
crystals. The remarkable phenomena exhibited during the 
transmission of heat along plates of glass, give additional 
weight to the deductions of analogy. Here the negative struc- 
ture invariably accompanies the positive structure, and the 
plates which are under the influence of these forces, exhibit 
phenomena precisely the same as artificial magnets.* It 
will, however, be naturally asked, what has become of the 
negative force in calcareous spar, if it is supposed to have two 
equal and rectangular positive axes ? To this we reply, 
that the negative force, originally less than the positive 
forces, has been balanced by the opposite actions of the two 
positive axes ; and that the resulting force, which has all 
the characters of a negative force, is merely the difference 
between the original negative force and the negative force 
which is equivalent to the effect of the positive axes. Nor 
is this mere speculation, for while it is an arrangement of the 
polarising forces which is as likely as any other to be the 
one actually followed by nature, it is almost directly sup- 
ported by a series of experiments, which I have made on 
plates and cylinders of glass.-f- 
* Pbil. Trans . 1816, p. 64, 83, 84, &c. 
4 See the Edinburgh Transactions, vol, vm. p. 353. 
