Mr B. Martin’s Experiments on Island Crystal. 25$ 
extent or bounds of this wonderful refracting power of Island 
crystal? Or by what method can we proceed to account for it? 
I once had a notion, that a number of fissures, discernible in 
some pieces, might be concerned in the cause of it ; but I am 
thoroughly satisfied now they are not. Sir Isaac Newton ac- 
counts for reflexion and reflradion of light by the action or 
power of an undulating fluid , or subtle spirit, near the surface 
of polished bodies ; and since it is more than probable that all 
the parts of natural bodies are kept in perpetual motion by the 
action of the same subtle spirit or fluid, why may we not, by a 
parity of reason, say that the different and multuple refractions 
in Island crystal, may be occasioned by the undulations within 
the surface, as well as without it ? If fits of easy refraction 
and easy reflexion of the rays , within their substance, under 
various modifications, will not be allowed, a more plausible rea- 
son ought to be assigned by those who can think on one that is 
so ; it is enough for me that I can prove the facts, and verify 
every thing here advanced, however strange and astonishing, by 
a variety of easy experiments. 
With respect to the figure of the several pieces of Island 
crystal, it is not to be supposed they are all of that regular 
form of an oblique parallelopiped , as represented in Fig. I of 
the Plate of the first Essay, though the generality of them are 
such. For this crystal spar, like many others, shoots out from 
a large base on the rock, into many irregular, uneven-sided 
scalenous pyramids , small and large ; which, when broken by 
gentle strokes with a hammer, readily split and slifter into many 
pieces, parallel-wise to the base ; and such as come out of the 
middle parts are of the figure above mentioned ; but those on 
the outside are of irregular forms, with opposite sides and angles 
unequal. Some are like a wedge or isosceles prisms , some qua- 
dr angular pyramids, &c. 
<c Again, some of these spars are very hard, and will take a 
fine polish ; others are softer, and though you may give them 
a polish, they will soon lose it. Some of these pieces are co- 
lourless and transparent as glass itself, though few of these are 
to be met with, as the greatest part is foul with mundic, and 
other opacious matter. Some pieces are of a yellowish hue, and 
others of a darker complexion ; all agree in having a double re- 
