LAWS OF MATTER AND MOTION. 



As no branch of science can be understood without some previous knowledge 

 of the general properties of matter, it will be desirable to commence by shortly 

 describing them : 



Extension is tne bulk of a body, its length, breadth, and thickness. 



Impenetrability is that property by which two bodies cannot at the same 

 time occupy one and the same place. If a nail be driven into a blocK of wood 

 it displaces the particles, but does not become incorporated with them (fig. 1). 

 If in a full glass of water a stone be placed, the water will be forced over to 

 make way for the stone (fig. 2). If we endeavour to fill a phial by plungiug 

 it into water, the air will rush out of the-%phial to make way for the water 

 (fig. 3). 



Divisibility denotes the property by which a body is susceptible of being 

 subdivided into an indefinite number of parts. Animalculse have been found so 

 small that a grain of sand will cover 300,000 of them, each one having a perfect 

 organization ; fig. 4 represents the forms of some highly magnified. 



Porosity arises from the influence which heat exercises in separating the 

 particles of matter. The piece of iron B, fig. 6, when cold, will exactly fit 

 into the hole and notch of A ; but if heated it will do neither. Fig. 7 repre- 

 sents the action of heat in expanding and setting in motion the particles of 

 water. 



Inertia or Persistence is the tendency of matter to preserve its present 

 state, whether of rest or motion, unchanged. Fig. 8 illustrates the first : if the 

 card be struck away the coin will remain balanced on the finger: fig. 9 illustrates 

 the second : a body in motion has a tendency to proceed in a straight line ; but 

 the hare being pursued by a dog, turns quickly, and the latter is irresistibly 

 thrown out of its track and compelled to take a wider turn, thus affording the 

 hare the only chance of escape. 



Cohesion is the force by which the atoms of a body are held together in one 

 solid mass. It is greater in some bodies than in others, the solidity or weight of 

 the body corresponding to the cohesive attraction. It is this po.wer which holds 

 the drop of water suspended at the end of the finger, and keeps its minute 

 watery particles united (fig. 10). Capillary attraction 1S another effect of 

 this power which enables liquids to rise above their level in minu-te tubes (fig .11) 

 Sap ascends in plants by the same force (fig. 12). 



Gravitation is that force which causes all bodies on or near the earth to tend 

 towards its centre with a force proportioned to their respective quantities of 

 matter (fig. 13). All bodies attract each other inversely as the squares of the 

 distances. All influences emanating from a central point follow the same law. 

 Fig. 14 illustrates the law in reference to light. At a certain distance the rays 

 illuminate the space A B ; at twice that distance they are spread over c D. four 

 times the former space, but with four times less intensity ; at three times the 

 distance they illuminate the space E F with nine times less intensity, and so on. 

 All bodies possess gravity or weight ; there is no such thing as perfect lightness. 

 Smoke ascends only because it is lighter than the atmosphere (fig. 15). The 

 force of gravity at the surface of the earth is such that in the first second of time, 

 it gives to a body allowed to fall, a velocity of 16 feet ; in the next second 48 

 feet ; in the third second 80 feet. Fig. 16 shows the rate at which bodies fall ; 

 each of the triangular portions representing 16 feet, the figures on the right, the 

 seconds. 



THE CENTRE OF GEAVITY. 



The centre of gravity in a body is that about which all its other parts equally 

 balance each other. Figs. 17 to 21 show the position of the centre of gravity in 

 bodies of different forms. The stability of a body resting on the ground depends 

 greatly upon the centre of gravity. The body will stand provided a vertical line 

 drawn from the centre of gravity falls within the base (figs. 22, 24). The mass of 

 rock (fig. 23) will fall because the vertical line falls beyond the base. Bodies 

 having a narrow base are easily upset, for if they are the least inclined their 

 centre is no longer supported, as seen in fig. 25. Rope dancers are provided 

 with a pole, loaded at the ends, for the purpose of bringing their centre of gravity 

 vertically over the rope (fig. 26). 



