depolarisation of light , &c. 39 
were put together, a partial depolarisation took place, arising 
probably from the obliquity of the folds. 
44. Glue depolarises light in every position. 
45. Hard isinglass depolarises light in every position. When 
dissolved in water, it acquires this property a few hours after 
coagulation. 
46. Acetate of lead. This salt melts at a temperature not 
much greater than that of bees’ wax, and takes a long time 
to cool and crystallize. It depolarises light in every position. 
When the plates of glass, in which it is included, are consi- 
derably inclined to each other, the acetate of lead developes 
a second image in the act of cooling, but owing to the cry- 
stallization which takes place, both the images are imperfectly 
visible. Its refractive power increases a little after crystalli- 
zation, and the new image that is developed is the one that is 
most refracted. 
47. Glass of borax. A thick piece of this glass depolarises 
light in every position. Another piece of considerable thick- 
ness had no effect upon polarised light. 
48. Amber sometimes depolarises light in every position, 
and sometimes exhibits neutral axes. 
49. Gum anime. A piece of this gum, three quarters of an 
inch thick, depolarises light in every position, and seems to 
produce the complementary colours by polarised light. Small 
fragments of it depolarise only a small quantity of light. 
50. Sulphur , when melted between two plates of glass, de- 
polarises light in every position. It acquires this property in 
a few minutes. 
51. Ice, Some plates of ice depolarise light in every posi- 
tion, while others exhibit neutral axes. 
