And that we may the better finde what the _cause_ of _Congruity_ and _Incongruity_ in bodies is, it will be requisite to consider, First, what is the _cause_ of _fluidness_; And this, _I conceive_, to be nothing else but a certain _pulse_ or _shake_ of _heat_; for Heat being nothing else but a very _brisk_ and _vehement agitation_ of the parts of a body (as I have elswhere made _probable_) the parts of a body are thereby made so _loose_ from one another, that they easily _move any way_, and become _fluid_. That I may explain this a little by a gross Similitude, let us suppose a dish of sand set upon some body that is very much _agitated_, and shaken with some _quick_ and _strong vibrating motion_, as on a _Milstone_ turn’d round upon the under stone very violently whilst it is empty; or on a very stiff _Drum_-head, which is vehemently or very nimbly beaten with the Drumsticks. By this means, the sand in the dish, which before lay like a _dull_ and unactive body, becomes a perfect _fluid_; and ye can no sooner make a _hole_ in it with your finger, but it is immediately _filled up again_, and the upper surface of it _levell’d_. Nor can you _bury_ a _light body_, as a piece of Cork under it, but it presently _emerges_ or _swims_ as ’twere on the top; nor can you lay a _heavier_ on the top of it, as a piece of Lead, but it is immediately _buried_ in Sand, and (as ’twere) sinks to the bottom. Nor can you make a _hole_ in the side of the Dish, but the sand shall _run out_ of it to a _level_, not an _obvious property_ of a fluid body, as such, but this dos _imitate_; and all this meerly caused by the vehement _agitation_ of the conteining vessel; for by this means, _each_ sand becomes to have a _vibrative_ or _dancing_ motion, so as no other heavier body can _rest_ on it, unless _sustein’d_ by some other on either side: Nor will it suffer any Body to be _beneath_ it, unless it be a _heavier_ then it self. Another Instance of the strange _loosening_ nature of a violent jarring Motion, or a strong and nimble vibrative one, we may have from a piece of _iron_ grated on very strongly with a _file_: for if into that a pin _screw’d_ so firm and hard, that though it has a convenient head to it, yet it can by no means be _unscrew’d_ by the fingers; if, I say, you attempt to unscrew this whilst _grated on by the file_, it will be found to undoe and turn very _easily_. The first of these Examples manifests, how a body actually _divided_ into small parts, becomes a _fluid_. And the latter manifests by what means the agitation of heat so easily _loosens_ and _unties_ the parts of _solid_ and _firm_ bodies. Nor need we suppose heat to be any thing else, besides such a motion; for supposing we could _Mechanically_ produce such a one _quick_ and _strong_ enough, we need not spend _fuel_ to _melt_ a body. Now, that I do not speak this altogether groundless, I must refer the Reader to the Observations I