I will now proceed to notice the principal effects of heat; viz., expansion, liquefaction, and vaporization.
Fig. 192.
274. Expansion in Solids.—Heat, you have seen in § 23, acts in opposition to the attraction of cohesion, tending to separate the particles, and so produces an expansion of any substance. This may be exemplified in the experiment represented in Fig. 192, in which A B is an iron rod, which is of such a size that at the ordinary temperature it will fit into the space, C D, in a bar of iron, and easily pass through the hole, E. If the rod be heated it will be enlarged or expanded in all directions, so that it will neither fit into C D nor pass into the hole, E. When the wheel-wright puts a tire upon a wheel he uses the expansion of heat to make it fit tightly and firmly. The tire is made a little too small to have it fit upon the wheel as it is. But by being heated it is so expanded that it will readily go on to the wheel, and then in contracting as it cools it so compresses the fellies as to hold on very tightly. Water is poured on to cool the iron quickly, and thus prevent it from burning the wood. Iron hoops are put on barrels in a similar manner, the compression caused by their contraction binding the staves together very strongly. So in fastening the plates of boilers together, the rivets are put in red-hot, so that in their contraction they may press the plates closely together. If an iron gate just shuts into its place in cold weather, its expansion will prevent its shutting when warm weather comes. In order to avoid this difficulty, calculation must be made in fitting it for its place for the expansion to which it will be subjected by heat. So in laying the rails of a railroad in cold weather care must be taken not to put the ends too near together. In constructing iron bridges the expansion by heat must be calculated for in the arrangement. Nails often become loose after the lapse of years from the wear of the wood around them, occasioned by their alternate expansion and contraction. The leaking of gas-pipes in the earth is often undoubtedly caused by the loosening of the joints from contraction and expansion of the pipes by varying temperatures of the soil, especially where they are not laid very deep. If a stopper stick fast in a bottle it can be loosened by the application around the neck of a cloth dipped in hot water, because the neck becomes expanded at once by the heat. A similar expedient was once very ingeniously made use of in repairing the machinery of the steamer Persia at sea, and was perhaps the means of saving the vessel and the lives of all on board. The accident which occurred was the breaking of the port crank-pin of the engine. The problem to be solved was the removal of this pin, which weighed nearly a ton, and the substitution of a sound one which they had on hand in its place. But it was found impossible to start the broken pin from its socket with all the force which could be brought to bear upon it by a sort of battering-ram constructed extemporaneously for the purpose. It was determined now to try the expansive force of heat. An iron platform was built under the socket, and a brisk fire made upon it. The socket soon expanded, and the pin was now readily knocked out by the battering-ram, just as the stopper of the bottle is easily removed when the neck is heated. The walls of a very large building in Paris, which had bulged out and were in danger of falling, were restored to their upright position by the expansion of heat.
Fig. 193.
It was done in this way: Long rods of iron were run through the walls after the plan represented in Fig. 193 (p. 213), their ends being made with a screw-thread, with nuts fitted to them. The rods marked a were first heated, and as they lengthened the nuts were screwed up tight to the walls. On cooling, their contraction would of course draw the walls together. The other bars, b, were now heated and managed in the same way. The one set, you see, were made to hold on by their nuts to what had already been gained, while the other were expanding. By many repetitions of this process the walls were righted and the building saved. The same mode has been adopted successfully in other cases of a similar character.
275. Expansion in Liquids.—Liquids are expanded by heat more than solids are. But they are very unequally expanded by it. Thus water is expanded more than twice as much as mercury, and alcohol six times as much. We have a frequent example of the expansion of water by heat in our kitchens. If the tea-kettle be put over the fire filled to the brim, it will run over long before the water begins to boil. All liquids occupy more space in summer than in winter, and in the former case weigh less—that is, have less of real substance in them than in the latter. If, therefore, alcohol, or oil, or molasses be bought by the gallon in winter and sold in summer, there will be a profit afforded by the expansion. Twenty gallons of alcohol in winter becomes twenty-one in mid-summer.
Fig. 194.