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the Caſe in earthen Veſſels, This is an Effect no way repugnant to Reaſon; for theſe are abſolutely dry, whereas whatever is fuſible muſt be, at leaſt in ſome Degree, moiſt; and retain, to the Time of its Fuſion, more or leſs of its Humidity.
ſible; for the ſame Stone, or even the very Powder, into which it has been ſhattered by the Fire, put into a Crucible with Salt of Tartar, or any other fixed alkaline Salt, and placed in a ſtronger Fire, will melt, and boil in the Veſſel; and form a very good Glaſs, as I have many Times experienced.
To learn the real Cauſes of the different Degrees of this Fuſibility in different foſſile Subſtances, it will be neceſſary, firſt, to conſider the Cauſe of their Solidity, or, in other Words, of their Coheſion: and this, as I have before obſerved, is that Power reſiding in all Matter, called Attraction.
This Power, it has already been obſerved, is infinitely ſtrongeſt at the Point of Contact: and therefore the Coheſion of all Bodies muſt be in Proportion to the Number of Points in which their conſtituent Particles touch one another. Thoſe Particles therefore which have the leaſt Solidity, with relation to their Surfaces, though they attract leaſt at Diſtances, yet, when they touch, cohere the moſt intimately; but where, from contrary Cauſes, the Coheſion is ſmall, as in ſpherical Bodies, whoſe Surfaces can only touch in a Point, their Particles eaſily recede from one another on any Impulſe; and whenever they are ſet in Motion, Fluidity takes Place.
By what means Fire is an Agent in bringing Things into this State, is eaſily underſtood. Its Particles, which are very powerful and very active, inſinuate themſelves into the Subſtance of the Matter to be melted, break and divide its Parts, and occaſion a much ſmaller Contact of them than there was before, and of Courſe a weaker Coheſion: more fiery Particles continually getting in as the Matter continues on the Fire; more and more diminiſh the Degree of Contact, till at laſt there is not enough of it to keep the Particles from rolling one over another, that is coming into a State of Fuſion.
This is the general Cauſe of the Fuſion of foſſile and other Subſtances; and the different
