Popular Scientific Recreations in Natural Philosphy, Astronomy, Geology, Chemistry, etc., etc., etc. Gaston Tissandier

be no rise in the latter. Extreme delicacy of the thermometer can be arrived at by using a very fine tube, particularly if it be also flat.

      The freezing point in Fahrenheit’s scale is 32°; in the Centigrade it is 0°, and the boiling point 100°. This was the scale adopted by Celsius, a Swede, and is much used. Réaumur called the freezing point 0°, and the boiling point 80°. There is another scale, almost obsolete—that of Delisle, who called boiling point zero, and freezing point 150°.

      There is no difficulty in converting degrees on one scale into degrees on the other. Fahrenheit made his zero at the greatest cold he could get; viz., snow and salt. The freezing point of water is 32° above his zero. Therefore 212–32 gives 180° the difference between the freezing and boiling points of water. So 180° Fahr. corresponds to 100° Cent., and to 80° Réaumur, reckoning from freezing point.

      The following tables will explain the differences:—

      

Table I.
1° Fahr.	= 0·55° Cent., or 0·44° Réaumur.
1° Cent.	= ·80° Réaumur, or 1·80° Fahr.
1° Réaumur	= 1·25° Cent., or 2·25° Fahr.

Table II.
	Fahr.	Cent.	Réaumur.
Boiling point	212	100	80
	194	90	72
	176	80	64
	158	70	56
	140	60	48
	122	50	40
	104	40	32
	86	30	24
	68	20	16
	50	10	8
Freezing point of water	32	0	0
	14	−10	−8
	−4	−20	−16
Freezing point of Mercury	−40	−40	−32

      Alcohol is used in thermometers in very cold districts, as it does not freeze even at a temperature of −132° Fahr.

      We have now explained the way in which we can measure heat by the expansion of mercury in a tube. We can also find out that solids and gases expand also. Engineers always make allowances for the effects of winter and summer weather when building bridges; in summer the bridge gets longer, and unless due provision were made it would become strained and weakened. So there are compensating girders, and the structure remains safe.

      The effects of expansion by heat are very great and very destructive at times. Instances of boilers bursting will occur to every reader. It is very important to be able to ascertain the extent to which solid bodies will expand. Such calculations have been made, and are in daily use.

      We can crack a tumbler by pouring hot water into it, or by placing it on the “hob.” A few minutes’ consideration will assure us that the lower particles of the glass expanded before the rest, and cracked our tumbler. A gradual heating, particularly if the glass be thin, will ensure safety. Thick glass will crack sooner than thin.

      Again, many people at railway stations have asked us, “Why don’t they join the rails together on this line?” We reply that if every length of rail were tightly fixed against its neighbour, the whole railway would be displaced. The iron expands and joins up close in hot weather. In wet weather, also, the wooden pegs and the sleepers swell with moisture, and get tightened up. Everyone knows how much more smoothly a train travels in warm, wet weather. This is due to the expansion of the iron and the swelling of the sleepers and pegs in the “chairs.” A railway 400 miles long expands 338 yards in summer—that is the difference in length between the laid railroad in summer and in winter.

      This can be proved. Iron expands 0·001235 of its length for every 180° Fahr. Divided by 180 it gives us the expansion for 1°, 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 temperature 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 contract 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 in the cases of Prince Rupert’s drops, and the Bologna flasks; the continuity 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.