392 Royal Society. 
is almost completely surrounded with a red-hot enclosure. It is also 
evident that, by placing the bomb in a dark room, we may view the 
transparent substance against a dark background. By this method 
of experimenting, therefore, the difficulty above alluded to is over- 
come. 
Before describing the experiment performed on tourmaline, it may 
be well to state what result the theory of exchanges would lead us to 
expect when this mineral is heated, and we shall perceive at the same 
time the importance of the experiment with tourmaline as a test of 
the theory. When a suitable piece of tourmaline, with its faces cut 
parallel to the axis, is used to transmit ordinary light, the hight which 
it transmits is nearly completely polarized, the plane of polarization 
depending on the position of the axis. The reason of this is, that if 
we resolve the incident light into two portions, one of which consists 
of light polarized in a plane perpendicular to the axis of the crystal, 
and the other of light polarized in a plane parallel to the same axis, 
nearly all the latter is absorbed, while a notable proportion of the 
former is allowed to pass. 
Suppose now that such a piece of tourmaline is placed in a red-hot 
enclosure; the theory of exchanges, when fully carried out, demands 
that the light transmitted by the tourmaline, say in a direction per- 
pendicular to its surface, plus the light radiated by the tourmaline 
in that direction, plus the small quantity of light reflected by the 
surface of the tourmaline in that direction, shall together equal in 
quantity and quality that which would have proceeded in the same 
direction from the wall of the enclosure alone, supposing the tour- 
maline to have been removed. Let us neglect the small quantity of 
light which is reflected from the surface of the tourmaline, and, 
standing in front of it, analyse with our polariscope the light which 
proceeds from it. This light consists of two portions, the trans- 
mitted and the radiated, both of which together ought to be equal 
in quality and intensity to that which would reach our polariscope 
from the enclosure alone were the tourmaline taken away. But the 
light which would fall on our polariscope from the enclosure alone 
would not be polarized; hence the whole body of light which falls 
upon it from the tourmaline, and which is similar in quality to the 
former, ought not to be polarized. Now part of this light, or that 
which is transmitted by the tourmaline, is polarized; hence it fel- 
lows, in order that the whole be without polarization, that the light 
which is radiated should be partially polarized in a direction at right 
angles to that which is transmitted. 
Another way of stating this conclusion is this. The light which 
the tourmaline radiates is equal to that which it absorbs, and this 
equality holds separately for light polarized in a plane parallel to the 
axis of the crystal, and for light polarized in a plane perpendicular 
to the same. , 
The experiment was made. with a piece of brown tourmaline 
having a few opake streaks, procured from Mr. Darker of Lambeth. 
It was placed in a graphite frame between two circular holes made 
as above described in opposite sides of the bomb, the diameter of the 
