304 Proceedings of Philosophical Societies. [APRIL 
nature of these investigations they could not be read to the Society, 
it is impossible to give any account of them. 
At the same meeting a paper by Dr. Brewster, on the means of 
giving the property of polarising light to glass, and common salt, 
and fluor spar, by pressure, was begun. The explanation of the 
doubly refracting property possessed by several bodies, notwithstand- 
ing the many new facts ascertained concerning the polarization and 
depolarization of light, still continued as difficult as ever; but 
some of the late observations of Dr. Brewster promise to throw 
new light on the subject. 
On Thursday the 7th of March Dr. Brewster’s paper was con-~ 
tinued. He showed that glass acquires the property of a crystallized 
body by being strongly pressed by means of a screw. A similar 
change is effected upon it by bending a plate of glass between the 
hands, the more strongly it is bent the greater is the effect which 
it produces on polarized light. He conceives, that in consequence 
of this property, new light will be thrown upon the effect of ex- 
ternal pressure in crushing or altering the structure of bodies, as 
arches, &c. 
On Thursday the 14th of March Dr. Brewster’s paper was con- 
cluded. Fluor spar, common salt, and other singly refracting 
bodies, may by compression be made to acquire the properties of 
doubly refracting crystals; but upon calcareous spar, sulphate of 
lime, and other refracting bodies, no change is produced by com- 
pression. Animal jellies, by compression, or dilatation, acquire 
the same properties as doubly refracting bodies. The author con- 
ceives, that a very sensible dynanometer may be constructed by 
means of glass, which is one of the most elastic of all bodies. A 
number of glass parallelopipeds fixed together may be bent by 
weights suspended from their middle, and by the changes in their 
effect upon polarized light, will indicate the degree of bending 
which they have undergone. By enclosing glass in fusible metal 
he conceives that very minute changes of temperature will be indi- 
cated by alterations in the density of the glass. Glass surrounded 
by a hygrometric substance will also act as a hygrometer. Dr. 
Brewster considers double refraction as probably resulting from the 
action of a peculiar fluid, and stated some circumstances which 
appeared favourable to that opinion. 
At the same meeting a paper by Mr. Babbage was announced, 
containing further observations on the theory of functions; but 
f rom the nature of the subject this paper could not be read. 
On Thursday the 21st of March a paper by Sir Everard Home 
was read, on the mode of action of specific medicines. From 
experiments already made it is known that poisonous bodies, whether 
mineral or vegetable, do not produce their effects upon the body 
till they are introduced into the circulation: and the effect always 
follows whenever they are introduced into the circulation. Ipeca- 
cuanna injected into the jugular vein produces instant vomiting, 
