FLUID, in physiology, an appellation given to all bodies, whose particles easily yield to the least partial pressure or force impressed.
All fluids, except those in the form of air or gas, are incompressible in any con siderable degree. The Academy del Ci mento, from the following experiment, supposed water to be totally incompressi ble. A globe made of gold, which is less porous than any other metal, was com pletely filled with water, and then closed up; it was afterwards placed under a great compressive force, which pressed the fluid through the pores of the metal, and formed a dew all over its surface, before any indent could be made in the vessel. Now, as the surface of a sphere will con tain a greater quantity than the same sur face under any other form whatever, the academy supposed that the compressive power which was applied to the globe must either force the particles of the fluid into closer adhesion, ordrive them through the sides of the vessel before any impres sion could be made on its surface ; for al though the latter effect took place, it fur nishes no proof of the incompressibility of water, as the Florentines had no method of determining that the alteration of figure in their globe of gold occasioned such a diminution of its internal capacity, as was exactly equal to the quantity of water forced into its pores ; but this expe riment serves to shew the great minute ness of the particles of a fluid in penetrat ing the pores of gold, which is the densest of all metals. Mr. Canton brought the question of incompressibility to a more decisive determination. He procured a glass tube, of about two feet long, with a ball at one end, of an inch and a quarter in diameter : having filled the ball and part of the tube with mercury, and brought it to the heat of 50° of Fahrenheit's ther mometer, he marked the place where the mercury stood, and then raised the mer cury by heat to the top of the tube, and there sealed the tube hermetrically ; then, upon reducing the mercury to the same degree of heat as before, it stood in the tube of an inch higher than the mark. The same experiment was repeated with water, exhausted of air, instead of mercu ry, and the water stood in the tube ,47;3, above the mark. Now, since the weight of the atmosphere on the outside of the ball, without any counterbalance front within, will compress the ball, and equally raise both the mercury and water; it ap pears that the water expands of an inch more than the mercury, by removing the weight of the atmosphere. From
this, and other experiments, he infers, that water is not only compressible, elastic ; and that it is more capable of compressibility in winter than in summer. All fluids gravitate, or weigh, in pro portion to their quantity of matter, not only in the open air, or in vacuo, but in their own elements. Although this law seems so consonant to reason, it has been supposed by ancient naturalists, who were ignorant of the equal and general pres sure of all fluids, that the component parts, or the particles of the same element, did not gravitate or rest on each other; so that the weight of a vessel of water balanc ed in air would be entirely lost, when the fluid was weighed in its own element. The following experiment seems to leave this question perfectly decided : take a common bottle, corked close, with some shot in the inside to make it sink, and fasten it to the end of a scale beam ; then immerse the bottle in water, and balance the weight in the opposite scale ; after wards open the neck of the bottle, and let it fill with water, which will cause it to sink ; then weigh the bottle again. Now it will be found that the weight of the water which is contained in the bottle is equal to the difference of the weights in the scale, when it is balanced in air ; which sufficiently shews that the weight of the water is the same in both situations. As the particles of fluids possess weight as a common property of bodies, it seems reasonable, that they should possess the consequent power of gravitation which belongs to bodies in general. Therefore, supposing that the particles which com pose fluids be equal, their gravitation must likewise be equal ; so that in the descent of fluids, when the particles are stopped and supported, the gravitation being equal, one particle will not have more propensity than another to change its situation, and after the impelling force has subsided, the particles will remain at absolute rest.