EXPANSION. 8 1 



yards in summer, that is the difference in length between the laid railroad 

 in summer and in winter. 



This can be proved. Iron expands croo 1235 of its length for every 1 80 

 Fahr. Divided by 1 80 it gives us the expansion for i, which is 0*00000686, 

 taking the difference of winter and summer at 70 Fahr. Multiply these 

 together, and the result (0*00048620 of its length) by the number of yards 

 in 400 miles, and we find our answer 338 yards. Expansion acts in solids 

 and most liquids by the destruction of cohesion between the particles. 

 Gases, however, having much less cohesion amid the particles, will expand far 

 more under a given heat than either solids or liquids, and liquids expand 

 more than solids for the same reason, and more rapidly at a high tem- 

 perature than at a low one. 



We have spoken of expansion. We may give an instance in which the 

 subsequent contraction of heated metal is useful. Walls sometimes get out 

 of the perpendicular, and require pulling together. No force which can be 

 conveniently applied would accomplish this so well as the cooling force due 

 to the potential energy of iron. Rods are passed through the walls and 

 braced up by nuts. The rods are then heated, and as they cool they con- 

 tract and pull the walls with them. 



When glass is suddenly cooled, the inner skin, as it were, presses with 

 great force against the cooled surface, but as it is quite tight no explosion 

 can follow. But break the tail, or scratch it with a diamond, and the strain 

 is taken off. The glass drop crumbles with the effect of the explosion, as 

 an the cases of Prince Rupert's drops, and the Bologna flasks ; the con- 

 tinuity is broken, and pulverization results. 



But a very curious exception to the general laws of expansion is 

 noticed in the case of nearly freezing water. We know water expands by 

 heat, at first gradually, and then to an enormous extent in steam. But when 

 cooling water, instead of getting more and more contracted, only contracts 

 down to 39*2 Fahr., it then begins to expand, and at the moment it freezes 

 into ice it expands very much about one-twelfth of its volume, but according 

 to Professor Huxley it weighs exactly the same, and the steam produced from 

 that given quantity of water will weigh just exactly what the water and the 

 ice produced by it weigh individually. At 39'2 Fahr. water is at its 

 maximum density, or in other words, a vessel of a certain size will hold 

 more water when it is at 39 Fahr. than at any other time. Whether the 

 water be heated or cooled at this temperature, it expands to the boiling or 

 freezing point when it becomes steam or ice, as the case may be. 



Water, when heated, is lighter than cold water. You can prove this in 

 filling a bath from two taps of hot and cold water at the same time. The 

 cold falls to the bottom, and if you do not stir up the water when mixed 

 you will have a hot surface and a cold foundation. The heat increases the 

 volume of water, it becomes lighter, and comes uppermost. 



Steam and Water and Ice are all the same things under different con- 

 ditions, although to the eye they are so different They are alike inasmuch 



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