PROCTOR, ON LIGHT. 159 
metals will be found by different observers to have colours 
different from those here attached to them. 
The antimony and arsenic were deposited from their com- 
binations with hydrogen, as usually practised by the analyst. 
Of course this metallic arsenic must not be confounded with 
its white oxide, which is known by the same name. The spe- 
cimens of charcoal are prepared from cork, pith, and common 
deal. Their tissues are not disintegrated by burning, and 
this affords us a ready means of obtaining amorphous carbon 
in thin films. In the deal charcoal the glandular deposits 
which characterise the vessels of coniferous trees may still be 
observed. 
Returning to our questions, “ What is the nature of proxi- 
mate opacity?” The illustrations which I have given show 
that most bodies transmit a coloured light, the colour deepen- 
ing as the thickness increases, until it is so dark that we 
call it opaque ; but there is no reason why all colours may 
not be absorbed equally, and then we have gray light trans- 
mitted. This is the general result, as might be expected, 
with a heterogeneous body consisting of particles of various 
refracting powers, and each individually of little absorbing 
power. In most instances we find the opacity caused by 
both a considerable absorbing power and the action of 
heterogeneousness, as explained when treating of the bundle 
of plates. 
It is more convenient to use words in their generally re- 
ceived meaning, therefore I will continue to call bodies opaque 
which, under ordinary circumstances, transmit no appreciable 
light ; and, with this qualification, again ask, “ Are all opaque 
bodies black when in a fine state of division ?”? The question 
was suggested by the fact that copper, deposited by electro- 
type, is of its usual colour if the current is of suitable inten- 
sity ; it becomes granular and purple brown if the current is 
somewhat too powerful, and becomes pulverulent and almost 
black if the current is very intense. Iron, reduced by 
chemical means from its oxide, without undergoing fusion, is 
bright, dark gray, or almost black, according to the degree of 
comminution in which it is obtained. Platinum obeys the 
same rule. Coke is a shining gray where the surface is 
smooth ; but when finely powdered, is black. So we might 
multiply mstances; but there are exceptions, and they prove 
that though it may be a rule it is not a law. I will only 
adduce one example, thatis, gold. Faraday has shown that it 
is a ruby red, a fine blue, purple, brown, &c., under different 
circumstances, but he did not obtain it black. 
The other two questions, regarding the blackness or 
