Having therefore in short set down my Notion of a Fluid body, I come in the next place to consider what _Congruity_ is; and this, as I said before, being a _Relative property_ of a fluid, whereby it may be said to be _like_ or _unlike_ to this or that other body, whereby it _does_ or _does not mix_ with this or that body. We will again have recourse to our former Experiment, though but a rude one; and here if we mix in the dish _several kinds_ of sands, some of _bigger_, others of _less_ and finer bulks, we shall find that by the agitation _the fine sand_ will _eject_ and _throw out_ of it self all those _bigger_ bulks of small _stones_ and the like, and those will _be gathered_ together all into _one_ place; and if there be _other_ bodies in it of other natures, those also will be _separated_ into a place by themselves, and _united_ or _tumbled_ up together. And though this do not come up to the _highest property_ of _Congruity_, which is a _Cohæsion_ of the parts of the fluid together, or a kind of _attraction_ and _tenacity_, yet this does as ’twere _shadow_ it out, and somewhat resemble it; for just after the same manner, I suppose the _pulse_ of heat to _agitate_ the small parcels of matter, and those that are of a _like bigness_, and _figure_, and _matter_, will _hold_, or _dance_ together, and those which are of a _differing_ kind will be _thrust_ or _shov’d_ out from between them; for particles that are _similar_, will, like so many _equal musical strings equally stretcht_, vibrate together in a kind of _Harmony_ or _unison_; whereas others that are _dissimilar_, upon what account soever, unless the disproportion be otherwise counter-ballanc’d, will, like so many _strings out of tune_ to those unisons, though they have the same agitating _pulse_, yet make quite _differing_ kinds of _vibrations_ and _repercussions_, so that though they may be both mov’d, yet are their _vibrations_ so _different_, and so _untun’d_, as ’twere to each other, that they _cross_ and _jar_ against each other, and consequently, _cannot agree_ together, but _fly back_ from each other to their similar particles. Now, to give you an instance how the _disproportion_ of some bodies in one respect, may be _counter-ballanc’d_ by a _contrary disproportion_ of the same body in another respect, whence we find that the subtil _vinous spirit_ is _congruous_, or does readily _mix_ with _water_, which in many properties is of a very _differing nature_, we may consider that a _unison_ may be made either by two _strings_ of the same _bigness_, _length_, and _tension_, or by two strings of the same _bigness_, but of _differing length_, and a _contrary differing tension_, or _3ly._ by two strings of _unequal length_ and _bigness_, and of a _differing tension_, or of _equal length_, and _differing bigness_ and _tension_, and several other such varieties. To which _three properties_ in _strings_, will correspond _three