refraction to glass , & c. by pressure . 1 73 
destroy the structure of the mass, the different tints are ar- 
ranged like those of the finest variegated marble, an effect 
exactly similar to what I have observed in numerous speci- 
mens of the diamond, and also in mixtures of rosin and white 
wax. 
By bringing the jelly into such a state that it is capable of 
being bent ; by coagulating it in glass troughs ; by applying 
dilating and compressing forces to a central point ; and by 
stretching it in thin elastic films over plates of compressed or 
dilated glass, a number of interesting results will be obtained. 
Proposition II. 
If a parallelopiped of jelly is allowed to indurate by exposure to 
the air , it will acquire at its edges a variable density, similar to 
that produced by pressure, and its edges will act upon light like 
doubly refracting crystals. 
Having poured some melted isinglass into a glass trough, 
and exposed it sometime after to polarised light, I observed a 
narrow and faint bluish stripe of the first order, on looking 
through the upper stratum. After a lapse of six hours, the 
tint became a brilliant white of the first order, and the stra- 
tum of jelly had depolarising axes inclined 4,5° to its length. 
In order to examine the mechanical change which the stratum 
had undergone, I looked through it at a small circular aper- 
ture. This aperture was elliptical, and its ellipticity gradually 
increased as the pencil passed nearer to the surface of the 
indurated stratum. Hence it follows, that the depolarising 
structure was produced or accompanied by a difference of 
density* 
