398 B. IL Woodward — Notes on Chemistry and Physics, 



gases, the motion is so great, that the particles are mutually repellent 

 within certain limits. 



That heat and motion are nearly related is clear enough, for we 

 cannot have one without the other. In bodies expanding with heat 

 we have motion ; and in the act of stopping a mo\'ing body heat is 

 generated by friction or concussion. 



It is one of the great axioms of modern physics that force cannot 

 be annihilated any more than matter. One variety of force may be 

 changed into another variety of force, but the absolute intensity re- 

 mains exactly equivalent. Thus if a body be allowed to fall, the 

 heat and other forces produced by the concussion would be just suf- 

 ficient to raise the body to exactly the same height from which it 

 fell, if the forces could be completely collected and economised. 



That heat will produce electricity is shown in the thermo-electric 

 pile, our most delicate measurer of heat. This instrument also shows 

 that electricity will produce motion by the repulsion of its index, a 

 freely suspended magnetic needle. Electricity was first noticed as a 

 force that was induced by motion — by the friction of amber — through 

 its power of again inducing motion in alternately attracting and re- 

 jjelling small fragments of light substances : the force expended, 

 being partly converted into heat and partly into electricity. 



When electricity is excited in any substance it is found that 

 another mode of force is always simultaneously called into play, 

 viz., magnetism, in a plane at right angles to the direction of the 

 electrical current. By this, magnetism shows that particles have 

 fixed and definite attitudes with regard to one another. 



If a magnet be moved across a body capable of conducting elec- 

 tricity transversely to the direction of the lines of magnetic force, 

 an electrical current is developed in that body, or if the magnet be 

 stationary and the conducting body moved in the same relative position 

 to the magnet, the same result is observed. Magnetism seems, in fact, 

 to be purely static ; it directs other forces, but does not initiate them, 

 unless motion be superadded. Through the medium of electricity it 

 can produce heat, light, and chemical affinity. Magnetism has a 

 most remarkable influence on light, for it causes the plane of polari- 

 zation to rotate while passing through water and several other 

 fluid media, which do not in this way alter the direction of the 

 ray under ordinary circumstances. 



We might adduce many more instances of this relation and inter- 

 dependence of the physical forces, showing their convertibility, 

 though this is not in all cases equally apparent : it is easy enough 

 to obtain light by means of heat, but the converse can only be 

 effected indirectly ; light will induce chemical action, for instance, 

 in a mixture of hydrogen and chlorine gases, and hence, in- 

 directly, heat. Yet so closely are they connected that it is 

 impossible to excite, produce, or disturb one force without calling 

 others into action. For instance, if sulphide of antimony be elec- 

 trified, magnetism is induced at right angles to the electrical current, 

 and it becomes heated and therefore expands, producing motion, and 

 if the electrical force be strong enough it becomes luminous and 



