NATURAL PHILOSOPHY MATTER, MOTION, AND HEAT. 



basin of water, while the other hangs over below 

 the level of the water, the basin will be emptied of 

 its contents ; and, on the same principle, when a 

 dry wedge of wood is driven into the crevice of a 

 rock, and afterwards moistened with water, it will 

 absorb the water, swell, and sometimes split the rock. 

 Endosmose or Osmose. Connected with capillary 

 attraction is endosmose. If two liquids be separated 

 by a piece of ox-bladder, the one below the mem- 

 brane being pure water, the other above being a 

 solution, say of sugar, the water will pass through 

 the membrane against gravity, and raise the solu- 

 tion above its former level. A smaller portion of 

 the solution finds its way into the water. This 

 remarkable phenomenon is known as endosmose 

 and exosmose, or simply as osmose. It is not yet 

 fully understood, but is believed to play a part 

 in the passage of the fluids through the mem- 

 branes of living animals and plants. 



Adhesion bet-ween Solids and Gases. In making 

 barometers, it is found that air adheres so firmly 

 to the surface of the glass, that the mercury must 

 be boiled in the tube before it can be expelled. 

 Some porous solids, such as charcoal, absorb air 

 and other gases to an amount many times their 

 own bulk, the force of adhesion condensing the 

 gases on the surface of their molecules. 



Chemical attraction is a force whose effects are 

 of a different kind from those of cohesion. If we 

 divide a piece of marble by breaking it into parts, 

 however small, each part is still marble. But the 

 chemist takes it to pieces in a different way. Out 

 of a piece of marble he will produce three distinct 

 substances, altogether unlike the original body a 

 metal not unlike silver, a black body called car- 

 bon, and a gas resembling air. Most of the sub- 

 stances of which our earth is made up are thus 

 composed of two or more different substances 

 or elements. Such unions are chemical unions ; 

 the ultimate particles that join to form them are 

 called atoms; and the force that binds them, 

 atomic force, or chemical attraction. The investi- 

 gation of changes that thus alter the constitution 

 of bodies, belongs to the science of CHEMISTRY. 



MOTION AND FORCES. 



Motion is change of place, and its opposite is 

 rest. Motion in any one body has always refer- 

 ence to the place of other bodies, and various 

 distinctive terms are used indicative of this refer- 

 ence. A man sitting on the deck of a ship has a 

 common motion with it ; if walking on the deck, 

 he has relative motion to the vessel. Absolute 

 motion means change of place with respect to 

 space itself. But we have no means of marking 

 a fixed point in space, and therefore can never 

 observe such a motion ; we know only relative 

 motions. As little do we know of absolute rest. 

 The earth is in constant rotation and also revolu- 

 tion round the sun ; and the sun himself is in 

 motion, we know not whither. Motion, and not 

 rest, is the great law of the universe. 



Velocity, or speed of motion, is measured by the 

 space passed over in a given unit of time ; as 

 when we say that a man walks three miles an 

 hour ; or that sound travels 1120 feet in a second. 

 The velocity is uniform, when equal spaces are 

 always passed over in equal times ; it is accelerated, 

 when gradually increased, and retarded, when 



gradually diminished. If the increase or diminu- 

 tion is equal in equal times, the motion is said to 

 be uniformly accelerated or uniformly retarded. 

 Force is any agency that produces or tends to 

 produce motion in a body, or to change or stop 

 its motion ; when the body is not free to move, 

 the force exerts a pressure. Force and pressure 

 are often used indifferently. The amount of a 

 pressure is measured by the weight it can support, 

 which is stated in pounds, ounces, &c. A force 

 causing motion is measured by the quantity of 

 motion or momentum it can communicate in a 

 given time. Momentum does not depend upon 

 velocity alone. Of two balls moving at the same 

 rate, but the one having twice the mass of the 

 other, the larger has just twice the momentum. 

 Momentum, then, is made up of velocity and 

 quantity of matter taken together. To get the 

 comparative momenta of two bodies in numbers, 

 we multiply the weight of each by its velocity. 



LAWS OF MOTION. 



The leading truths respecting the movements of 

 bodies have been summed up in the shape of a 

 few axioms, or propositions, which were first put 

 into shape by Newton under the name of Laws 

 of Motion. We shall give them as laid down by 

 Newton, and then follow them up with observa- 

 tions on each. 



i st. Every body continues in its state of rest, or 

 of uniform motion in a straight line, except in so 

 far as it may be compelled to change that state 

 by forces impressed upon it. 



2d. Change of motion is proportional to the 

 impressed force, and takes place in the direction 

 of the straight line in which the force is impressed. 



3</. To every action there is always an equal 

 and contrary reaction ; or the actions of two 

 bodies upon each other are always equal, and 

 oppositely directed. 



FIRST LAW. 



This law is little else than a definition of the 

 property of Inertia or Inactivity. There are three 

 things to be attended to in this law namely, the 

 persistency of matter, both in rest and motion, 

 when not acted upon by external agency ; the 

 uniform velocity; and the straight direction of 

 motion when once begun and left to itself. The 

 first has been already illustrated under the head 

 of Inertia ; and when we say that unobstructed 

 motion is naturally uniform, we are only repeating 

 in a different form the same truth. 



A little reflection brings out the truth of the 

 third point namely, that motion is naturally 

 straight. If a ball projected forward is seen to 

 bend to the right or the left, we infer at once that 

 something interfered with it. That it bends down- 

 wards, we know to be owing to the attraction of 

 the earth. Motion therefore requires force to 

 bend it 



SBCOND LAW. 



That every change of motion is proportional to 

 the force that produces it, is involved in the way 

 of measuring force, which is by the quantity of 

 motion it gives or takes away. As to the direction 

 of the resulting motion, the law requires careful 

 explanation. When the body is previously at 

 rest, it is self-evident that, if unobstructed, it will 

 follow the direction of the force that begins to act 



