32 Dr. Brewster’s experiments on the 
one about half of the light, and one nearly the whole of the 
light. 
Semi-opal , like the diamond, depolarises light in every posi- 
tion ; but there is an obvious approximation to neutral axes. 
In employing another class of bodies, we are presented 
with a series of very singular results, which not only develope 
new affections of light, but lead to important conclusions respect- 
ing the crystallization of organised and unorganised bodies. 
1 . Gum Arabic. This vegetable substance, which is formed 
by concentric coats, has no neutral axes ; but depolarises light 
in every position. In a very thin chip, however, the neutral 
axes are distinctly visible. With a strong solution of gum 
Arabic in water, I formed a thin film of it upon a plate of 
glass. In two or three days it became very hard, but though 
I have kept it six months, it has not acquired the property of 
depolarising light. 
2. Gum from the cherry tree. A plate of this gum about one- 
twentieth of an inch thick, and so soft as to yield to the gen- 
tlest pressure of the nail, depolarises light, but the image does 
not wholly vanish in the neutral axes. Another plate of the 
gum, but exuded from a different part of the same tree, and 
much softer than the former, depolarises only a small por- 
tion of light. 
3. Caoutchouc. This gum is composed of concentric or pa- 
rallel layers of a vegetable juice, which are successively in- 
durated by exposure to the sun. When a thin plate of it is 
made transparent by pressure between two plates of glass, it 
exhibits no neutral axes, but depolarises light in every po- 
sition, in whatever direction the film is cut from the mass. 
