, are the small interstices between the particles of matter which compose bodie<*>; and are either empty, or filled with some insensible medium.

Condensation and rarefaction are only performed by closing and opening the Pores. Also the transparency of bodies is supposed to arise from their Pores being directly opposite to one another. And the matter of insensible perspiration is conveyed through the Pores of the cutis.

Mr. Boyle has a particular essay on the porosity of bodies, in which he proves that the most solid bodies have some kind of Pores: and indeed if they had not, all bodies would be alike specisically heavy.

Sir Isaac Newton shews, that bodies are much more rare and porous than is commonly believed. Water, for example, is 19 times lighter and rarer than gold; and gold itself is so rare, as very readily, and without the least opposition, to transmit magnetic effluvia, and easily to admit even quicksilver into its pores, and to let water pass through it: for a concave sphere of gold hath, when filled with water, and soldered up, upon pressing it with a great force, suffered the water to squeeze through it, and stand all over its outside, in multitudes of small drops like dew, without bursting or cracking the gold. Whence it may be concluded,| that gold has more pores than solid parts, and consequently that water has above 40 times more Pores than parts. Hence it is that the magnetic effluvia passes freely through all cold bodies that are not magnetic; and that the rays of light pass, in right lines, to the greatest distances through pellucid bodies.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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PORTA (John Baptista)