NATURAL PHILOSOPHY. 169 



In cities and enterprising towns there are systems of water- 

 pipes and gas-conductors, of metal, ramifying in tlie interior of 

 dwellings and other structures. Such buildings should be cai-e- 

 fully protected 'outside. If the conducting medium, whether of 

 water or gas-pipes, preponderates in the interior of the building, 

 the electric fluid may leave the external conductor, and through a 

 thick wall seek that which facilitates its passage to the earth. In 

 such cases, it seems that nothing but a rod, having numerous 

 points for collecting the electricity and adequate means for convey- 

 ing it innocuously to the earth, would be an etfectual protection. 

 Some authorities recommend a connection to be made between 

 the system of water and gas-pipes inside a building and the ex- 

 ternal conductor. — Scientijic American. 



ALL THINGS IN MOTION. 



In imagining the ultimate composition of a solid body, we have 

 to reconcile two apparently contradictory conditions. It is an 

 assemblage of atoms which do not touch each other, — for we are 

 obliged to admit intermolecular spaces, — and yet those atoms 

 are held together in clusters by so strong a force of cohesion as 

 to give to the whole the qualities of a solid. This would be the 

 case even with a solid undergoing no change of size or internal 

 constitution. But solids do change, under pressure, impact, heat,^ 

 and cold. Their constituent atoms are, consequently, not at rest. 

 Mr. Grove tell us: "Of absolute rest, nature gives us no evi- 

 dence. All matter, as far as we can ascertain, is ever in move- 

 ment, not merel}' in masses, as with the planetary spheres, but 

 also molecularl}^ or throughout its most intimate structure. 

 Thus, every alternation of temperature produces a molecular 

 change throughout the whole substance heated or cooled. Slow 

 chemical or electrical actions, actions of light or invisible radiant 

 forces, are always at play ; so that, as a fact, we cannot predicate 

 of any portion of matter, that it is absolutely at rest." 



The atoms, therefore, of which solid bodies consist are supposed 

 to vibrate, to oscillate, or better, to revolve, like the planets, in 

 more or less eccentric orbits. Suppose a solid body to be rc^jre- 

 sented by a swarm of gnats dancing in the sunshine. Each gnat 

 or atom dances up and down at a certain distance from each other 

 gnat, within a given limited space. The path of the dance is not 

 a mere straight line, but a vertical oval — a true oi'bit. Suppose, 

 then, that, in consequence of greater sun heat, the gnats become 

 more active, and extend each its respective sweep of flight. The 

 swarm, or solid body, as a whole, expands. If, from a chill or the 

 shadow of a cloud, the insect's individual range is less extensive, 

 the crowd of gnats is necessarily denser, and the swarm, in its 

 integrity, conti-aets. 



Tyndall takes for his illustration a bullet revolving at the end 

 of a spiral spring. He had sj^oken of the vibration of tlie mole- 

 cules of a solid as causing its expansion, but he remarks that, by 

 some, the molecules have been thought to revoh'e round each 

 otlier ; the communication of heat, by augmenting their centrif- 

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