49 
depolarisation of light , &c. 
When the first layer of gum Arabic or caoutchouc is de- 
posited and crystallized, it will possess both neutral and depo- 
larising axes like every other crystal. The second layer will 
likewise have these axes, but there is manifestly no cause 
which can make the neutral and depolarising axes of the 
second layer coincide with those of the first layer ; so that 
after a number of layers are formed, there will be a depola- 
rising axis in every direction. In order to illustrate this con- 
clusion by direct experiment, we have only to place one plate 
of mica above another, so as to make the neutral axis of the 
one coincide with the depolarising axis of the other. It will 
then be seen that all the axes become depolarising axes, and 
that the compound crystal acts upon light exactly like gum 
Arabic and caoutchouc. If this explanation be correct, we 
should expect to find, that a film of gum Arabic or caoutchouc, 
reduced to a less thickness than any individual layer, would 
exhibit its neutral axes, and lose the property of depolarising 
light in every position. This interesting result I have re- 
peatedly obtained both with gum Arabic and caoutchouc, as 
described in experiments i and 3, so that we have both a 
synthetical and an analytical proof of the explanation which 
has been given of the third kind of depolarisation. 
Hence it follows, that all substances which depolarise light 
in every position, are formed by layers successively deposited 
and crystallized ; that every layer has neutral and depolaris- 
ing axes, like regularly crystallized bodies ; and that the axes 
of one layer is not related in point of direction to those of its 
adjacent layers. 
4. Th e fourth kind of depolarisation is exhibited in the film of 
gold beaters’ skin, where there is an approach to a neutral axis, 
MDCCCXV. H 
