VOL. X.j PHILOSOPHICAL TRANSACTIONS. 217 



In the next place, weigh also in the air and water a piece of copper or brass, if 

 that be the likeliest to be enoployed in counterfeiting the coin, and observe 

 their difference : then the less of these differences being subtracted from the 

 greater, the remainder will show how much the true piece of coin will outweigh 

 the other in water ; and consequently if so many grains as this remainder 

 amounts to, being added to the weight of the lighter metal, do make a suffici- 

 ently manifest depression of it below the mark, it would stay at without that 

 addition, we may conclude, that probably the difference between a true and 

 counterfeit piece of coin proposed, will be discoverable by the instrument : but 

 it may be expedient for those that have frequent occasion to examine various 

 sorts of coin, to have a several instrument adjusted for each of them. 



With this instrument pure tin may certainly be distinguished from such as is 

 adulterated: for as gold, being the heaviest of metals,* cannot be allayed with 

 any other, that will not depress the instrument less than gold can do; so tin 

 being the lightest of metals, cannot be mixed with any other, that will not sink 

 lower than unmixed tin; still supposing the weight to be the same in the air. 



In the same manner may pewter be compared and examined. For having 

 once observed how much the instrument is depressed by a piece of two, three, 

 or four drams, or even an ounce weight of pewter, which is known to be good, 

 and to contain such a proportion of lead in reference to the tin; if you load the 

 instrument with an equally heavy piece of any other mass of pewter proposed, 

 and the instrument sink deeper, it will be a sign that the former proportion of 

 lead may be very probably argued to exceed in the mixture. This instrument 

 may also assist in making a pretty tolerable estimate of the fineness of gold, and 

 its different allays with silver, or some other determinate metal. In order to 

 which the instrument may be fitted to sink to the tip of the pipe, with some 

 determinate weight of the finest gold, as of 24 carats. But it will be proper, 

 that this metal in the air be some determinate weight, that is commodiously 

 divisible into many aliquot parts. Then you may make a mixture that contains 

 a known proportion of the metal wherewith you allay the gold : as, if it contain 

 J 9 or 15 parts of gold, and one of silver: and letting the instrument settle in 

 the water, mark the place where the surface of the water cuts the stem or pipe. 

 Then putting in another mixture, wherein the silver has a new and greater pro- 

 portion to the gold ; as if the former be an 18th or 14th part of the latter, you 

 may observe how much less this depresses the instrument. And thus you may 



• Gold was the heaviest of the then known metals ; but platina, (afterwards discovered, and of 

 which an account will be given in a future volume of these Transactions) surpasses it_, when freed frona 

 all its impurities, in specific gravity. 



VOL. II, F F 



