378 Mr. A. Mallock [May 27, 



thousandth part of its volume by a pressure of 1 lb. per square inch, 

 and for solids, such as metals, the compression would be much less. 



The resistance to compression of fluids is easily shown, but the 

 corresponding resistance to dilation, though it exists, is more trouble- 

 some to demonstrate by experiment, owing to the difficulty of 

 securing the adhesion of the fluid to the surface of the vessel which 

 contains it. 



Experiments have been shown in this room in which water and 

 mercury, free from air and placed in perfectly clean vessels, have 

 borne large negative pressures, and the increment of volume which 

 can be brought about in this way is limited by the want of adherence 

 to boundaries rather than by the want of coherence in the liquids 

 themselves. There is, however, a limit to the coherence of all liquid 

 and solid matter when the volume is forcibly increased, and this 

 limit is reached when the expansion is a very small fraction of the 

 original volume. 



In gases, on the other hand, there is no coherence, and the 

 particles instead of holding together naturally tend to separate, a 

 fact which is well accounted for by what is known as the molecular 

 theory. 



According to this theory, all the molecules of which the gas is 

 made up are in continual motion, generally striking and rebounding 

 from one another or from the boundaries which enclose them after a 

 short free path. Pressure in a gas depends on the effect of the sum 

 of these continual impacts, and thus not only on the velocity of the 

 molecules, but also on the total number of molecules in a definite 

 volume — that is, on the density. Temperature, on the other hand,, 

 depends on the average velocity only, the average velocity being 

 taken as the square root of the mean of the squares of the velocities 

 of all the molecules engaged. For air at ordinary temperatures and 

 at atmospheric pressures this velocity is somewhat in excess of 1,600 

 feet per second. 



The relation between pressure, volume and temperature to which 

 the molecular theory leads is : 



Pressure x Volume = Absolute Temperature X a Constant, 



and this, taken as it stands, would indicate that if the temperatures 

 were kept constant there would be no limit to the reduction of 

 volume with the increase of pressure. The simple form of the 

 theory takes no account of the size of the molecules themselves, and 

 only represents the facts so long as the compression is insufficient to 

 bring them into close contact with one another. When this contact 

 does happen the variation of volume with pressure no longer depends 

 on the reduction of the space between molecules, but on the com- 

 pression of the actual molecules themselves, and there is every 

 reason to believe that no force, however large, can reduce the volume 

 of matter below a certain limit. 



