33 
depolarisation of light, &c. 
When the film was so extremely thin, that it could not be 
made transparent by pressure between two plates of glass, I 
stretched it over the mouth of a tube upon a piece of plane 
glass, and prevented it by means of a cord from recovering its 
shape. A layer of Canada balsam being then placed both below 
and above the caoutchouc, and another plate of glass laid upon 
the upper layer of balsam, the film became perfectly trans- 
parent. When exposed to polarised light, it exhibited neutral 
axes, like the most perfect crystals. 
When caoutchouc is dissolved by heat, it loses completely 
its property of depolarisation, but it gradually recovers its 
former structure, and after a certain number of days, it is 
again capable of depolarising light. A piece of caoutchouc, 
which had been melted by heat, resumed its faculty of depo- 
larisation at the end of twenty-five days ; but upon pressing 
it gently with my finger, its structure was again destroyed, 
and at the end of nineteen days, it depolarised a small por- 
tion of nebulous light. Another piece of caoutchouc, dissolved 
by heat, had not recovered its crystalline state at the end of 
six days. After standing eighteen days, it depolarised a con- 
siderable quantity of light, and at the end of five or six weeks, 
it was capable of restoring the whole of the vanished image. 
4. White wax. When a piece of wax is melted and cooled 
between two plates of glass, or when it is merely pressed 
between them by the heat of the hand, it depolarises light in 
every position. The restored image, however, has a nebu- 
lous appearance. 
g. Rosin and white wax mixed. Rosin alone has not the pro- 
perty of depolarising light. When it is mixed with an equal 
part of white wax, and is pressed between two plates of glass 
mdcccxv. F 
