COMPRESSION
, the act of pressing, or squeezing something, so as to bring its parts nearer together, and make it occupy less space.
Compression differs from condensation as the cause from the effect, compression being the action of any force on a body, without regarding its effects; whereas condensation denotes the state of a body that is actually reduced into a less bulk, and is an effect of compression, though it may be effected also by other means. Nevertheless, compression and condensation are often confounded.
Pumps, which the ancients imagined to act by suction, do in reality act by compression; the piston, in working in the narrow pipe, compresses the inclosed air, so as to enable it, by the force of its increased elasticity, to raise the valve, and make its escape; upon which, the balance being destroyed, the pressure of the atmosphere on the stagnant surface, forces up the water in the pipe, thus evacuated of its air.
It was long thought that water was not compressible into less bulk: and it was believed, till lately, that after the air had been purged out of it, no art or violence was able to press it into less space. In an experiment made by the Academy del Cimento, water, when violently squeezed, made its way through the fine pores of a globe of gold, rather than yield to the compression.
But the ingenious Mr. Canton, attentively considering this experiment, found that it was not sufficiently accurate to justify the conclusion which had always been drawn from it; since the Florentine philosophers had no method of determining that the alteration of sigure 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. To bring this matter therefore to a more accurate and decisive trial, he procured a small glafs 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 exactly to the heat of 50° of Fahrenheit's thermometer, he marked the place where the mercury stood in the tube, which was about six inches and a half above the ball; he then raised the mercury by heat to the top of the tube, and there sealed the tube hermetically; then upon reducing the mercury to the same degree of heat as before, it stood in the tube 32/100 of an inch higher than the mark. The same experiment was repeated with water exhausted of air, instead of mercury, and the water stood in the tube 43/100 of an inch above the mark. Since the weight of the atmosphere on the outside of the ball, without any counterbalance from within, will compress the ball, and equally raise both the mercury and water, it appears that the water expands 11/100 of an inch more than the mercury by removing the weight of the atmosphere. Having thus determined that water is really compressible, he proceeded to estimate the degree of compression corresponding to any given weight. For this purpose he prepared another ball, with a tube joined to it; and finding that the mercury in 23/100 of an inch of the tube was the hundred thousandth part of that contained in the ball, he divided the tube accordingly. He then filled the ball and part of the tube with water exhausted of air; and leaving the tube open, placed this apparatus under the receiver of an air-pump, and observed the degree of expansion of the water answering to any degree of rarefaction of the air: and again by putting it into the glass receiver of a condensing engine, he noted the degree of compression of the water corresponding to any degree of condensation of the air. He thus sound, by repeated trials, that, in a temperature of 50°, and when the mercury has been at its mean height in the barometer, the water expands one part in 21740; and is as much compressed by the weight of an additional atmosphere; or the compression of water by twice the weight of the atmosphere, is one part in 10870 of its whole bulk. Should it be objected, that the compressibility of the water was owing to any air which it might be supposed to contain, he answers, that more air would make it more compressible; he therefore let into the ball a bubble of air, and found that the water was not more compressed by the same weight than before.
In some farther experiments of the same kind, Mr. Canton found that water is more compressible in winter than in summer; but he observed the contrary in spirit of wine, and oil of olives.
The following table was formed, when the barometer was at 29 inches and a half, and the thermometer at 50 degrees.
| Compression of | Millionth parts. | Spec. grav. |
| Spirit of wine | 66 | 846 |
| Oil of olives | 48 | 918 |
| Rain water | 46 | 1000 |
| Sea water | 40 | 1028 |
| Mercury | 3 | 13595 |
The compression of the air, by its own weight, is surprisingly great: but the air may be still further compressed by art. See Elasticity of Air.
This immense compression and dilatation, Newton observes, cannot be accounted for in any other way, but by a repelling force, with which the particles of air are endued; by virtue of which, when at liberty, they mutually fly each other.
This repelling power, he adds, is stronger and more sensible in air, than in other bodies; because air is generated out of very fixed bodies, but not without great difficulty, and by the help of fermentation: now those particles always recede from each other with the greatest violence, and are compressed with the greatest difficulty, which, when contiguous, cohere the most strongly. See Air, Attraction, Cohesion, DILATATION, and Repulsion.
