Page:Theophrastus - History of Stones - Hill (1774).djvu/67

 ; for the ame Stone, or even the very Powder, into which it has been hattered by the Fire, put into a Crucible with Salt of Tartar, or any other fixed alkaline Salt, and placed in a tronger Fire, will melt, and boil in the Veel; and form a very good Glas, as I have many Times experienced.

To learn the real Caues of the different Degrees of this Fuibility in different foile Subtances, it will be neceary, firt, to conider the Caue of their Solidity, or, in other Words, of their Coheion: and this, as I have before oberved, is that Power reiding in all Matter, called Attraction.

This Power, it has already been oberved, is infinitely tronget at the Point of Contact: and therefore the Coheion of all Bodies mut be in Proportion to the Number of Points in which their contituent Particles touch one another. Thoe Particles therefore which have the leat Solidity, with relation to their Surfaces, though they attract leat at Ditances, yet, when they touch, cohere the mot intimately; but where, from contrary Caues, the Coheion is mall, as in pherical Bodies, whoe Surfaces can only touch in a Point, their Particles eaily recede from one another on any Impule; and whenever they are et in Motion, Fluidity takes Place.

By what means Fire is an Agent in bringing Things into this State, is eaily undertood. Its Particles, which are very powerful and very active, ininuate themelves into the Subtance of the Matter to be melted, break and divide its Parts, and occaion a much maller Contact of them than there was before, and of Coure a weaker Coheion: more fiery Particles continually getting in as the Matter continues on the Fire; more and more diminih the Degree of Contact, till at lat there is not enough of it to keep the Particles from rolling one over another, that is coming into a State of Fuion.

This is the general Caue of the Fuion of foile and other Subtances; and the different the Cae in earthen Veels, This is an Effect no way repugnant to Reaon; for thee are abolutely dry, whereas whatever is fuible mut be, at leat in ome Degree, moit; and retain, to the Time of its Fuion, more or les of its Humidity.