MATTER AND ITS PHYSICAL PROPERTIES. 33 



air which is enclosed in the bladder from the pressure of the external air. When 

 this is accomplished, the bladder will be observed to swell as if it were in- 

 flated, and will become perfectly distended. The air contained in it, there- 

 fore, has a tendency to dilate, which takes effect when it ceases to be resisted 

 by the pressure of surrounding air. 



It has been stated that the increase or diminution of temperature is accom- 

 panied by an increase or diminution of volume. Related to this there is anoth- 

 er phenomenon, too remarkable to pass unnoticed, although this is not the proper 

 place to dwell upon it : it is the converse of the former, viz., that all increase 

 or diminution of bulk is accompanied by a diminution or increase of tempera- 

 ture. As the application of heat from some foreign source produces an increase 

 of dimensions, so, if the dimensions be increased from any other cause, a cor- 

 responding portion of the heat which the body had before the enlargement will 

 be absorbed in the process, and the temperature will be thereby diminished. 

 In the same way, since the abstraction of heat causes a diminution of volume, 

 so, if that diminution be caused by any other means, the body will give out the 

 heat which in the other case was abstracted, and will rise in its temperature. 



Numerous and well-known facts illustrate these observations. A smith, by 

 hammering a piece of bar iron, and thereby compressing it, will render it red 

 hot. When air is violently compressed, it becomes so hot as to ignite cotton 

 and other substances. An ingenious instrument for producing a light for do- 

 mestic uses has been constructed, consisting of a small cylinder, in which a 

 solid piston moves air tight ; a little tinder, or dry sponge, is attached to the 

 bottom of the piston, which is then violently forced into the cylinder. The air 

 between the bottom of the cylinder and the piston becomes intensely com- 

 pressed, and evolves so much heat as to light the tinder. 



In all the cases where friction or percussion produces heat or fire, it is be- 

 cause they are means of compression. The effects of flints of pieces of wood 

 rubbed together the warmth produced by friction on the flesh are all to be 

 attributed to the same cause. 



INERTIA. 



The quality of matter which is of all others the most important in mechani- 

 cal investigations, is that which has been called inertia. 



Matter is incapable of spontaneous change. This is one of the earliest and 

 most universal results of human observation; it is equivalent to stating that 

 mere matter is deprived of life ; for spontaneous action is the only test of the \ 

 presence of the living principle. If we see a mass of matter undergo any | 

 change, we never seek for the cause of that change in the body itself ; we look , 

 for some external cause producing it. This inability for voluntary change of 

 state or qualities is a more general principle than inertia. At any given mo- 

 ment of time, a body must be in one or other of two states, rest or motion. In- 

 ertia, or inactivity, signifies the total absence of power to change this state. A < 

 body endued with inertia cannot of itself, and independent of all external influ- S 

 ence, commence to move from a state of rest ; neither can it, when moving, 

 arrest its progress, and become quiescent. 



The same property by which a body is unable by any power of its own to 

 pass from a state of rest to one of motion, or vice versa, also renders it inca- 

 pable of increasing or diminishing any motion which it may have received from 

 an external cause. If a body be moving in a certain direction at the rate c 

 ten miles per hour, it cannot, by any energy of its own, change its rate of mo- 

 tion to eleven or nine miles an hour. This is a direct consequence 

 manifestation of inertia which has just been explained. For the same power , 



VOL.. II. 3 ^^~>. ^~^ 



