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on that subject. Sir Isaac Newton made trial of a vetf 

 small portion of it, and found the same elects from it as 

 I experienced: but it is possible, that, if it were added in 

 the just proportion discovered by Mr. Edwards, it would 

 be an improvement, and useful ingredient, in the compo- 

 sition.* 



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

 tarnish from the air, is otherwise susceptible of it acciden- 

 tally; when there happen to be minute holes in its surl'ace, 

 caused by the air, or sand, &c. in casting it. Svich cavities 

 will be filled with the dust, or rusty solutibn of the brass, in 

 grinding; which Avill, in time, become a sort of vitrioh 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 drying, while it remains heated, to rub it over 

 ^vith spirit-varnish; from which it may be cleansed, by a 

 piece of fine linen dipped in spirit of wine. The varnish Avill 

 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 



* Having rea^ somewhere, that zint and gold made the best speculum-metal, 

 I ti'ied it; and found, that the zinc wis sublimed from the gold in fusion, 

 and arose to the top in the crucible, forming a white, hard, spongy mass. 

 The metal, called tutanag, is fit for specula, when melted with tin; but I am 

 certain, that what I procured, under the name of tutanag, was a mixture of 

 brass and copper, &c. ; for the zinc, in the brass, rose frofn it, during the 

 fftsion, in white flowers. 



