One reason, which seems to have led us into the mistake that the _solidity_, or _firmness_ of bodies is not caused by the _air_, has been, that, for the generality, we consider the _air_, or _atmosphere_ as _pressing_ only _downwards_; for if we had considered that it _presseth_ equally _every way_, as well as _downwards_, (as _Boerhaave_ in his chym. by _Shaw_, vol. 1. p. 389. has shew by the following experiment) I believe we should not have overlooked that force, or have thought it insufficient for this operation. “Fill three glass vessels, the one of a cylindrical figure, the other conical, the third bellied with a cylindrical neck; let these be filled to the brim with fair water, and covered with a single piece of paper, so as to touch the surface of the water, and by pressing it down with the hand, prevent the external air insinuating between the paper and the water; if the glasses be now inverted, whilst the paper remains close with the palm of the hand, and the hand be afterwards gently withdrawn, the water will still remain in the glasses. The same holds true, though the glasses be held horizontal, or in any other position.” As the cause of the _solidity_ or _firmness_ of bodies is the _pressure_ of the _air_ or _atmosphere_, so likewise it must depend on the _make_ and _size_ of the _pores_ of such bodies; for bodies whose _pores_ are _smallest_, must be acted upon with a greater power than those whose _pores_ are _largest_, or whose _pores_ are so _large_ as not only to admit _light_, but also _common air_ into them. This Mr. _Hauksbee_ proves by a curious experiment, for having placed two brass hemispheres, of 3-1/2 inches diameter, upon each other, and then extracting the gross air out of them by the air pump, and by these means taking off the _resistance_ of the _common air_ that was _within_ the two brass hemispheres, he says, it required 140 pound weight to separate them; this experiment with that of the two marble slabs before mentioned, is a demonstration of the _power_ that keeps _solid bodies_ from _falling_ to _pieces_. And even, if these slabs are not so _perfectly smooth_, yet the weting them with water, which prevents the gross air from _entering_, will produce the like effect. That solids _expand_ themselves by _heat_ or _fire_, is proved by heating an iron rod in the fire: in which case, it is always found to be _bigger_ and _longer_ when _hot_ than _cold_; and it was the opinion of _Boerhaave_, that cold _consolidates_ all those that are called firm bodies; that is, brings that part, which we call _body_ in them, into a _less compass_ than before, and thus _unites_ the matter thereof more closely together: by which means the _cohesion_ of the whole mass is usually increased; which makes what we call, _strength_ and _firmness_ in bodies.