FLUIDITY

, that state or affection of bodies, which denominates or renders them Fluid; or that property by which they yield to the smallest force impressed: in contradistinction to Solidity or Firmness.

Fluidity is to be carefully distinguished from Liquidity or Humidity, which latter implies wetting or adhering. Thus, air, ether, mercury, and other melted metals, and even smoke and flame itself, are Fluid bodies, but not Liquid ones; whilst water, beer, milk, urine, &c, are both Fluids and Liquids at the same time.

The nature and causes of Fluidity have been variously assigned. The Gassendists, and ancient corpuscularians, require only three conditions as necessary to it; viz, a smallness and smoothness of the particles of the body, vacuities interspersed between them, and a spherical figure. The Cartesians, and after them Dr. Hook, Mr. Boyle, &c, beside these circumstances, require also a certain internal or intestine motion of the particles as chiefly contributing to Fluidity. Thus, Mr. Boyle, in his History of Fluidity, argues from various experiments: for example, a little dry powder of alabaster, or plaister of Paris, finely sifted, being put into a vessel over the fire, soon begins to boil like water; exhibiting all the motions and phenomena of a boiling liquor: it will tumble variously in great waves like that; will bear stirring with a stick or ladle like that, without resisting; and if strongly stirred near the side of the vessel, its waves will apparently dash against it: yet it is all the while a dry parched powder.

The like is observed in sand; a dish of which being set on a drum-head, briskly beaten by the sticks, or on the upper stone of a mill, it in all respects emulates the properties of a Fluid body. A heavy body, ex. gr. will immediately sink in it to the bottom, and a light one emerge to the top: each grain of sand has a constant vibratory and dancing motion; and if a hole be made in the side of the dish, the sand will spin out like water.

The Cartesians bring divers considerations to prove that the parts of Fluids are in continual motion: as 1st, The change of solids into Fluids, ex. gr. ice into water, and vice versa; the chief difference between the body in those two states consisting in this, that the parts, being fixed and at rest in the one, resist the touch; whereas in the other, being already in motion, they give way to the slightest impulse. 2dly, The effects of Fluids, which commonly proceed from motion: such are the insinuation of Fluids among the pores of bodies; the softening and dissolving hard bodies; the actions of corrosive menstruums; &c: Add, that no solid can be brought to a state of Fluidity, without the intervention of some moving or moveable body, as fire, air, or water. Air, the same gentlemen hold to be the first spring of these causes of Fluidity, it being this that gives motion to fire and water, though itself receives its motion and action from the ether, or subtle medium.

But Boerhaave pleads strenuously that fire is the first mover, and the cause of all Fluidity in other bodies, as air, water, &c: without this, he shews that the atmosphere itself would fix into one solid mass. And in like manner, Dr. Black, of Edinburgh, mentions Fluidity as an effect of heat. The different degrees of heat which are required to bring different bodies into a state of Fluidity, he supposes may depend on some particulars in the mixture and composition of the bodies themselves: which is rendered farther probable from considering that the natural state of bodies in this respect is changed by certain mixtures; thus, when two metals are compounded, the mixture is commonly more fusible than either of them separately.

Newton's idea of the cause of Fluidity is different: he makes it to be the great principle of attraction. The various intestine motion and agitation among the particles of Fluid bodies, he thinks is naturally accounted for, by supposing it a primary law of nature, that as all the particles of matter attract each other when within a certain distance; so at all greater distances, they avoid and fly from one another. For then, though their common gravity, together with the pressure of other bodies upon them, may keep them together in a mass, yet their continual endeavour to avoid one another singly, and the adventitious impulses of heat and light, or other external causes, may make the particles of Fluids continually move round about one another, and so produce this quality.

As therefore the cause of cohesion of the parts of solid bodies appears to be their mutual attraction; so, on this principle, the chief cause of Fluidity seems to be a contrary motion, impressed on the particles of Fluids; by which they avoid and fly from one another, as soon as they come at, and as long as they keep at, such a distance from each other.

It is observed also in all Fluids, that the direction of their pressure against the vessels which contain them, is in lines perpendicular to the sides of such vessels; which property, being the necessary result of the spherical sigure of the particles of any Fluid, shews that the parts of all Fluids are so, or of a figure very nearly approaching to it.