135 
Specula • 
I must observe here, that a metal, not liable to contract 
tarnish from the air, is otherwise susceptible of it acci- 
dentally : when there happen to be minute holes in its 
surface, caused by the air, or sand, &c. in casting it. Such 
cavities will be filled with the dust, or rusty solution of 
the brass, in grinding ; which will, in time, become a 
sort of vitriol, and act on the contiguous parts of the 
speculum, producing a canker in it, which will spread 
in form of a cloud of tarnish, around each cavity. In 
such a case, to prevent this, I would advise, to lay the 
mirror, as soon as polished, in warm water, and, after dry- 
ing, while it remains heated, to mb it over with spirit- 
varnish, from which it may be cleansed, by a piece of fine 
linen dipped in spirit of wine. The varnish will remain 
in the cavities ; and, by defending the impurities in them 
from the action of the air, will probably preserve them 
from becoming corrosive to the metal. 
From numerous experiments, of the qualities of differ- 
ent compositions, made by several persons, it appears, 
that no combinations, of any other metals or semi-metals, 
are fit for specula, except those of copper, brass, tin, sil- 
ver, and arsenic. I tried no semi- metal, except the lat- 
ter, which whitens copper, and unites intimately with it : 
because it is stated, in the treatise of the Art of Assaying ■, 
by the observant and accurate Cramer ; that all the semi- 
metals rise in flowers, during the fusion : which would 
certainly make the metal porous. On this account, I 
would have rejected the brass, because of the zinc con- 
tained in it ; but that it seemed to render the composition 
whiter, and less apt to tarnish, than it would be without ffc 0 
It will have little tendency to rise in flowers, if the specu- 
lum, metal be fused, with the lowest heat requisite, and 
if the brass be of the best kind; because, in this, the zinc 
is more perfectly united with the copper, and both are 
purer* I used, for dps purpose, the brass of pin* wire : 
