( 3 47 ) 
be the proportion of the two Ingredients, the weight of the 
Mafs in the Air be juft the fame with that of the pure Gold, 
which we have lately fuppofed to be one Ounce , or half an 
Ounce. 
By the fame method may be examined the differing Alloys of 
pure Silver upon the admixture of fach and fuch determinat 
proportions of Goppcr or any other Metal lighter in fpecie 
than Sliver; and by the fame way, with a flight variation , 
'twill not be difficult to eftimate , how much divers Coins, 
whether of Silver or Gold, are more or lefs embas'd by the 
known ignobler Metal that is mixt in the piece propos'd. 
And though this way of determining the Alloys of MetalJs 
be notfoexaft as is polfible to be proposed by the help of Hy- 
droftaticks and Calculation 5 yet it may be very ufeful ta 
Chymifts, Goldfmiths, Refiners, and others, that are unac» 
quainted with Hydroftatical matters, to make without trouble 
or fupputation eftimates that will not much deceive them, 
and perhaps will come nearer the Truth , not only than the 
eflimaies wont to be made by the Touchflone, but perhaps^ 
too , than fome of thofe that divers make with trouble, and 
inconvenience, and charge. And indeed I was chiefly invi- 
ted to communicate this Trifle and fpend fo many words a- 
bout it by the requeflof fome Ingenious Difciples of Vulcan , 
who thought ihey perceived, that, by this way, they could 
oftentimes make better eftimates of thefuccefsof their gradua- 
ting , and fome other , operations upon Metals , than other- 
wife they fliould be able , this way greatly accommodating 
them by this particular advantage, that they may fiom time 
to time try the degrees of purity, and fome other confiderable 
alterations of their mixtures, without at all deflroying or in- 
juring them , though they havenot yet attained the pitch rhey 
aim at and expeft ; whereas, if they happen to be too forward, 
as often they are, in examining the Produftions of their Labours 
by the cupel or fevere Cementations, what they would try 
may bedeflroyed orfpoilt in its way to a perfeflion, which 
Z 2 2 other- 
