201 
Dr. Brewster on the laws of polarisation, &c, 
thus received, we ought carefully to consider whether the Huy- 
genian law, upon which it rests, is the universal law of double 
refraction, or merely an elegant and correct expression of 
the individual phenomena of calcareous spar. Because this 
law represents with accuracy the action of a single crystal, is 
it thence to be presumed that all the other transparent 
crystals of the mineral kingdom possess the same structure, 
and exhibit the same series of appearances - as calcareous 
spar P We venture to say that such a conclusion is scarcely 
possible in the present state of science ; and that if it should 
happen to have been rashly drawn, it could not be delibe- 
rately supported by any one who is imbued with the cautious 
spirit of the inductive philosophy. 
It will be replied, however, to this question, that Malus 
has examined, with the utmost care, the properties of quartz , 
arragonite , and sulphate of barytes ; that he has demonstrated 
the identity of their action with that of calcareous spar ; and 
that the extension of the law of Huygens to other crystallized 
bodies can no longer be doubtful. This reply would have 
possessed considerable weight, had the alledged identity of 
action been satisfactorily established ; but the experiments of 
Malus are decidedly erroneous, for out of the three crystals, 
the identity of whose action with that of calcareous spar he 
is supposed to have demonstrated, no fewer than two have 
more than one axis of double refraction. 
Here then is a difficulty of an unexpected kind : for if the 
experiments of Malus on these crystals are set aside as in- 
correct, what confidence can we place in his observations on 
calcareous spar, upon which the truth of the Huygenian law 
is founded ? If nitre and arragonite, both of which have a 
MDCCCXVIII. D d 
