32 



MATTER AND ITS PHYSICAL PROPERTIES. 



are, however, not absolutely incompressible, but yield slightly to very intense 

 pressure. 



The question of the compressibility of liquids was raised at a remote period 

 in the history of science. Nearly two centuries ago an experiment was insti- 

 tuted at the Academy del Cimento in Florence, to ascertain whether water be 

 compressible. With this view, a hollow ball of gold was filled with the liquid, 

 and the aperture exactly and firmly closed. The globe was then submitted to 

 a very severe pressure, by which its figure was slightly changed. Now, it is 

 proved in geometry that a globe has this peculiar property, that any change what- 

 ever in its figure must necessarily diminish its volume or contents. Hence it was 

 inferred that if the water did not issue through the pores of the gold, or burst the 

 globe, its compressibility would be established. The result of the experiment 

 was that the water did ooze through the pores, and covered the surface of the 

 globe, presenting the appearance of dew or of steam cooled by the metal. But 

 this experiment was inconclusive. It is quite true that if the water had not 

 escaped, upon the change of figure of the globe, the compressibility of the liquid 

 would have been established. This escape of the water does not, however, 

 prove its incompressibility . To accomplish this, it would be necessary first to 

 measure accurately the volume of water which transuded by compression, and 

 next to measure the diminution of volume which the vessel suffered by its 

 change of figure. If this diminution were greater than the volume of water 

 which escaped, it would follow that the water remaining in the globe had been 

 compressed, notwithstanding the escape of the remainder. But this could 

 never be accomplished with the delicacy and exactitude necessary in such an 

 experiment, and consequently, as far as the question of the compressibility of 

 water was concerned, nothing was proved. It forms, however, a very striking 

 illustration of the porosity of so dense a substance as gold, and proves that its 

 pores are larger than the elementary particles of water, since they are capable 

 of passing through them. 



It has since been proved that water and other liquids are compressible. In 

 the year 1761, Canton communicated to the Royal Society the results of some 

 experiments which proved this fact. He provided a glass tube with a bulb, 

 like that of a common thermometer, and filled the bulb and a part of the tube 

 with the liquid well purified from air. He then placed this in an apparatus 

 called a condenser, by which he was enabled to submit the surface of the liquid 

 in the tube to a very intense pressure of condensed air. He found that the 

 level of the liquid in the tube fell in a perceptible degree upon the appli- 

 cation of the pressure. The same experiment established the fact that liquids 

 are elastic ; for, upon removing the pressure, the liquid rose to its original level, 

 and therefore resumed its former dimensions. 



Elasticity does not always accompany compressibility. If lead or iron be 

 submitted to the hammer, it may be hardened and diminished in its volume, 

 but it will not resume its former volume after each stroke of the hammer. 



There are some bodies which maintain the state of density in which they 

 are commonly found by the continual agency of mechanical pressure, and such 

 bodies are endued with a quality in virtue of which they would enlarge their 

 dimensions without limit if the pressure which confines them were removed. 

 Such bodies are called elastic fluids or gases, and always exist in the form of 

 common air, in whose mechanical properties they participate. They are hence 

 called aeriform fluids- 



Those who are provided with an air-pump can easily establish this property 

 experimentally. Take a flaccid bladder and place it under the glass receiver 

 of an air-pump. By this instrument we shall be able to remove the air which 

 surrounds the bladder under the receiver so as to relieve the small quantity of 



