502 PHILOSOPHICAL TRANSACTIONS. (]aNNO I7O9. 



dissipate and fly away in fume or small particles. The reason of which I take 

 to be this : charcoal is a substance deeply impregnated with oily or sulphureous 

 particles. The first effect that fire has upon metals is to separate the sulphureous 

 parts ; now, if in proportion as the sulphur is separated from the metal, the 

 body that supports the metal furnishes it anew with other sulphureous parts, the 

 other principles will never separate, and the metal will always remain metal ; 

 and nothing but the greatest degree of fire can raise and separate the sulphur, 

 and that but by little and little, and in very small particles. 



I then had recourse to another matter, which could by no means be sus- 

 pected of containing any oily parts. Mr. Tschirnhaus, to whom we are in- 

 debted for making these large glasses, and the first experiments that have been 

 made with them, says, he has vitrified metals by holding them in China ware. 

 It is true, this succeeds pretty well, provided the pieces be very thick, and the 

 glazing taken off. Of all the different kinds of matter that I tried, what seem 

 best were the common cupels, and plates of grey fire- stone. The cupels 

 hold the metal a long time in fusion in the focus of the glass without melting; 

 excepting lead, which easily runs through them as soon as it vitrifies, and helps 

 to dissolve them. The plates of fire-stone bear the heat of the focus much 

 longer than any other matter, but great care is to be taken in heating them, 

 without breaking, till they become red-hot ; for when they are hot, the least 

 cold air makes them melt. 



The experiments on iron were the following : I placed in the focus of the 

 burning-glass a piece of forged iron, of about a dram weight ; it became red- 

 hot, and its surface was covered with a black matter, like pitch or tar. On 

 withdrawing the iron out of the focus in this state, this matter fixes itself on the 

 surface of the metal, and there forms a small skin or very fine blackish scale, 

 which is easily separated by striking upon it ; and that part of the iron that was 

 covered with this scale appears blacker than ordinary. This scale is some of 

 the sulphureous part of the iron, which rises to the surface of the metal when 

 ready to melt, and there remains for some time, before it exhales. It is plainly 

 this sulphureous part that rises upon iron, and polished steel, when heated, and 

 gives them all those different colours, from a yellow to a violet, a water co- 

 lour, or a black. By continuing to hold this piece of iron on the charcoal, it en- 

 tirely melts, and at the same time emits very bright sparks in great quantities, 

 sometimes to more than a foot distance from the coal. By saving what flies 

 off during this sparkling, by holding a sheet of paper under the coal, we find 

 that they are so many very small globules of iron, and the greatest part of them 

 hollow. 



All the iron, that is held in fusion on the coal, flies away in sparkles after 



