CHAMBERS'S INFORMATION FOR THE PEOPLE. 



varying in number and arrangement in different 

 substances ; these groups are called molecules, 

 and bodies are held to be built up of aggregations 

 of such molecules. 



But whether the ultimate component particles 

 of bodies have a fixed size and shape or not, we 

 know that they are indestructible. This is not, 

 indeed, what a first impression suggests, for noth- 

 ing is more common than for bodies to decay, 

 dissolve, evaporate, and -disappear. But it can be 

 proved that in no case is anything lost. Water 

 disappears in invisible vapour when heated; but 

 if the vapour is carefully collected and cooled, 

 the water reappears without loss of weight. 

 The substance of the coal burned in our fires 

 is not annihilated; it is only dispersed in the 

 form of smoke or particles of soot, gas, and ashes 

 or dust 



Inertia or Inactivity. The term inertia or in- 

 activity is meant to express the fact, that an 

 inorganic body has no power to change its state. 

 If it is at rest, it cannot put itself in motion ; if it 

 is in motion, it cannot bring itself to rest; any 

 change must come from some external cause. It 

 hardly gives a correct notion to say that bodies 

 are quite passive to a change of state ; for they 

 resist the change, with a force depending upon 

 the mass of matter they contain and the amount 

 of motion sought to be given to them or taken 

 away. No one term conveys all that is meant ; 

 persistence has been suggested as less objection- 

 able than inertia, inactivity, or passiveness. 



The following instances illustrate the action of 

 this property of matter : A great force is neces- 

 sary at first to set a vehicle in motion ; but when 

 once this is effected, it goes onward with com- 

 parative ease ; so. that, in fact, a strong effort is 

 necessary before it can be stopped. If a person 

 be standing in it when it is suddenly set agoing, 

 his feet are pulled forward, whilst his body, obey- 

 ing the law of inertia, remains where it was, and 

 he accordingly falls backwards. On the other 

 hand, if the vehicle be suddenly stopped, and the 

 individual be standing in the same position as 

 formerly, the tendency which his body has to 

 move forward for it acquired the same motion 

 as the carriage by which it was borne along will 

 cause him to fall in that direction. A man jump- 

 ing from a coach at full speed falls prostrate on 

 the ground ; for his body has the same motion 

 as the coach ; and when the feet arrive at the 

 ground, they are arrested, while the rest of the 

 body moves on ; and thus he finds himself thrown 

 down. 



The process of beating a carpet, or dusting a 

 book, rests on the same principle. When a dusty 

 book is struck against a table, the book and the 

 dust are first brought into rapid motion together, 

 and the book being then arrested by the table, 

 the dust continues in motion by its inertia, and is 

 thus detached. 



It is a common impression with those who have 

 never reflected upon the subject, that bodies are 

 more inclined to rest than to motion. This arises 

 from the fact, that while no body begins to move 

 or increase its speed without some cause for the 

 change being apparent, all the motions that come 

 within our observation on the surface of the earth 

 do actually come to an end, most of them gradu- 

 ally, and without any very apparent cause. Thus 

 the notion is begotten, that rest is the natural state 



194 



to which all matter, when left to itself, seeks, as it 

 were, to return. But a little consideration shews 

 us that the retardation and stoppage of motion are 

 as dependent on causes as its beginning is. A 

 ball rolled on the rough earth soon stops ; on a 

 wooden floor, it continues longer ; and on smooth 

 ice, longer still. This shews that one cause of the 

 arrest of terrestrial motions is friction. Another 

 constant impediment is the resistance of the air. 

 A pendulum set in motion in an exhausted re- 

 ceiver, will continue to swing, without the help of 

 clock-work, for a whole day, having nothing to 

 resist its motion but the small amount of friction 

 at its point of suspension. Finding thus that 

 motion is prolonged in proportion as we diminish 

 obstructions to it, though we can never completely 

 remove them, we conclude, that if they were 

 removed, motion once begun would go on for ever. 



It is in the heavenly bodies, however, that we 

 find complete proof of this truth. They move 

 without friction, and unresisted by any fluid mani- 

 fest to the senses, and no appreciable slackening 

 of their speed has yet been detected ; they re- 

 tain the amount of motion they had from the 

 beginning. 



Compressibility, Contractibility ; Expansibility, 

 Dilatability. When a body is forced by mechani- 

 cal pressure into less space than it previously 

 occupied, it is said to be compressed; when any 

 cause not mechanical, such as loss of heat, causes 

 its volume to diminish, it is said to be contracted. 

 Expansion and dilatation are used to express 

 enlargement of volume or bulk. Now, all bodies 

 whatever are liable to these two opposite kinds of 

 change without any addition to, or deduction 

 from, the matter composing them. Even those 

 substances which we consider as types of solidity 

 are compressible. A piece of iron, when squeezed 

 in a vice or hammered, loses in bulk, and be- 

 comes more compact. 



The most compressible substances are air and 

 other gases. A moderate pressure will force a 

 quantity, of air confined in a vessel into half its 

 volume ; and as the pressure is increased, the 

 volume goes on diminishing almost without limit. 

 By pressure and cold combined, several gases 

 have been reduced to the liquid form. Compared 

 with gases, or even solids, liquids have little 

 compressibility. (See HYDROSTATICS.) 



Expansion or dilatation is chiefly seen in the 

 case of heat being applied to bodies (see page 204). 

 Gases are unlimitedly dilatable. (See HYDRO- 

 STATICS and PNEUMATICS.) 



Elasticity. Some bodies, when compressed, re- 

 cover their former size when the pressure is 

 removed. Such bodies are called elastic; and 

 those which remain as the compressing force put 

 them, are non-elastic. Air and other gases afford 

 the best examples of elasticity. Caoutchouc, 

 ivory, and steel are among the most elastic of 

 solid substances. But no solid body is perfectly 

 elastic, nor are any completely non-elastic ; so 

 that elasticity may be considered as general a 

 property of matter as compressibility. The cause 

 of elasticity is not well understood. 



Porosity and Density. In common language, a 

 pore is a small hollow space or interstice between 

 the particles of a body, large enough to be seen, 

 or to admit the passage of liquids or gases. In 

 this sense, some substances, such as sponge, 

 wood, sugar, &c. are called porous, and others 



