;52i REFLECTING TELESC6PE. 



deterred from employing it : and unless it shall be found that, 

 without this effect, it makes the metal less porous than other- 

 wise it might be, or less frail and brittle, I am certain that i^ 

 may, in every other respect, be dispensed with. 1 had no op- 

 portunity to try it, in the precise quantity Mr. Edwards recom- 

 mends, (though I did so before, in very nearly that proportion,) 

 since I first saw his memoir on that subject. Sir Isaac New- 

 ton made trial of a very small portion of it, and found the same 

 effects from it as I experienced : but it is possible, thaJ, if it 

 were added in the just proportion discovered by Mr. Edwards, 

 it would be an improvement, and useful ingredient, in the com- 

 position *. 



I must observe here, that a metal, not liable to contract tar- 

 liable to tarnish *^^5h from the air, is otherwise susceptible of it accidentally; 

 from imperfect when there happen to be minute holes in its surface, caused by 

 mg. ^j^^ ^j^^ ^^ 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 tainish, around each 

 cavity. In such a case, to prevent this, I would advise, to lay 

 the mirror, as soon as polished, in waim water, and, after 

 drying, while it remains healed, to rub it over with spirit-var- 

 nish , from which it may be cleansed, by a piece of fine linen 

 dipped in spirit of wine. The varnish will remain in the cavi. 

 ties ; 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 diflferent com- 

 tion, copper, positions, made by several persons, it appears, that no combi- 



br3s$, tin, sil- nations, of any other metals or semi-metals, are fit for specula, 

 Ver, and ar- ' ^ 



^enic. 



* Having read somewhere, that zinc and gold made the best 

 speculum-ractal, I tried it ; and found, that tiie zinc was sublimed 

 from the gold in fusion, and arose to she top in the crucible, 

 for-ming a white, hard, spongy mass. The metal, called tutanag, 

 is fit for specula, when melted with tin ; but I am certain, that what 

 I procued, under the name of tutanag, was a mixture of brass 

 and copper, &cc. ; for the zinc, in the brass, rose from it, duringtli|f 

 fusion, in white flowers. 



