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

of water is equal to the atmospheric pressure of 15 lbs. on the square inch, and thus at such a distance water must be heated to 234° before it will boil. Professor Bunsen founded his Theory of the Geysers upon this fact, for he maintained that water falling into the earth lost much air, and required with the super-incumbent pressure a very high temperature to boil it. When it did boil it generated steam so suddenly that it exploded upwards, throwing up vapour and the water with it, as water poured into a very hot basin will do.

      Evaporation may now be considered, and is distinguished from Ebullition by the production of vapour on the surface of liquids, the latter term signifying the formation of vapour in the body of the liquid. Evaporation takes place at all temperatures, and from every liquid surface exposed to the air. We know what we call a “drying wind.” The air in fresh layers continually passing over the wet ground, takes up the moisture; like the east wind, for instance, which has great capabilities of that nature. Damp air can only take up a certain quantity, and when it contains as much water as corresponds to the temperature it can take no more, and is “saturated with moisture”; then evaporation ceases. Heat is a great cause of evaporation, and the greater the surface the more rapid the process, and in a vacuum more readily than in atmospheric air. Evaporation is resorted to very commonly to produce coolness; for instance, the universal fan, by increasing evaporation from a heated skin, generates a feeling of coolness; and we know the vaporization of ether will freeze into insensibility. When a fluid evaporates we can tell that the heat passes away at the same time, for we cool water in porous jars, which permit some of it to pass off in vapour, the remainder being cooled.

      Sir John Leslie invented a method of freezing water by rapid evaporation on sulphuric acid under the receiver of an air-pump, and water has been frozen even on a hot plate by these means. By pouring sulphurous acid and water on this plate, the acid evaporates so quickly that it produces sufficient cold to freeze the water it quitted into solid ice.

      We leave the phenomena of clouds and watery vapour in the atmosphere for consideration on another opportunity, under the head of Meteorology, Rain, etc.

      Fig. 80.—Apparatus for freezing carafes of water.

      An experiment is often performed by which water is frozen in a vacuum. By putting a saucer full of water under the receiver of an air-pump it will first boil, and then become a solid mass of ice. It is not difficult to understand the cause of this. The water boils as soon as the air is removed; but in order to pass from the liquid to the gaseous state without the assistance of exterior heat, it gives out heat to the surroundings, and in so doing becomes ice itself. This fact Mr. Carré has made use of in the apparatus shown above (fig. 80). A small pump creates a vacuum in the water bottles, and ice is formed in them.

      This apparatus might easily be adopted in country houses, and in places where ice is difficult to procure in summer. The only inconvenience attending it is the employment of sulphuric acid, of which a considerable quantity is used to absorb the vapour from the water, as already referred to. If proper precautions are taken, however, there will be no danger in using the apparatus.

      The mode of proceeding is as follows:—The bottle full of water is joined to the air-pump by a tube, and after a few strokes the water is seen in ebullition. The vapour thus disengaged traverses an intermediate reservoir filled with sulphuric acid, which absorbs it, and immediately condenses it, producing intense cold. In the centre of the liquid remaining in the carafe some needles of ice will be seen, which grow rapidly, and after a few more strokes of the pump the water will be found transformed into a mass of ice. This is very easy of accomplishment, and in less than a minute the carafe full of water will be found frozen.

      The problem for the truly economical formation of ice by artificial means is one of those which have occupied chemists for a long time, but hitherto, notwithstanding all their efforts, no satisfactory conclusion has been arrived at. Nearly every arrangement possesses some drawback to its complete success, which greatly increases the cost of the ice, and causes inconvenience in its production. The usual mode in large towns is to collect the ice, in houses constructed for the purpose, during the winter, and this simple method is also the best, so far as at present has been ascertained.

      Fig. 81.—Retort and Receiver.

      In connection with vaporization we may now mention two processes referred to just now (page 83); viz., sublimation and distillation. The former is the means whereby we change solid bodies into vapour and condense the vapour into proper vessels. The condensed substances when deposited are called sublimates, and when we go into Chemistry we shall hear more of them. The mode of proceeding is to place the substance in a glass tube, and apply heat to it. Vapour will be formed, and will condense at the cool end of the tube. The sublimate of sulphur is called “Flowers of Sulphur,” and that of perchloride of mercury “Corrosive Sublimate.”

      Distillation is a more useful process, or, at any rate, one more frequently employed, and is used to separate a volatile body from substances not volatile. A distilling apparatus (distillo, to drop) converts a liquid to vapour by means of heat, and then condenses it by cold in a separate vessel.

      The distilling apparatus consists of three parts—the vessel in which the liquid is heated (the still, or retort), the condenser, and the receiver. The simple retort and receiver are shown in fig. 81. But when very volatile vapours are dealt with, the arrangement shown on next page is used (fig. 82). Then the vapour passes into the tube encased in a larger one, the intervening space being filled with cold water from the tap above (c), the warm water dropping from g. The vapours are thus condensed, and drop into the bottle (or receiver) B.

      Fig. 82.—Distilling apparatus.

      The apparatus for distilling spirits is shown below. The “still” A is fitted into a furnace, and communicates with a worm O in a metal cylinder filled with water, kept constantly renewed through the tube TT′. This spirit passes through the spiral, and being condensed, goes out into the receiver C.

      Fig. 83.—Spirit still.

      There are even more simple apparatus for spirit distilling, but the diagram above will show the principle of all “stills.” In former days, in Ireland, whiskey was generally procured illegally by these means.

      

CHAPTER VIII.

       Table of Contents

      SPECIFIC HEAT—FUSION—LATENT HEAT—CONDUCTION AND CONVECTION OF HEAT—CALORESCENCE.

      We have considered the effects of heat upon water, and touched upon one or two kindred experiments. But we have some other subjects to discuss, two in particular; viz., Specific Heat, and Latent Heat.

      The specific heat of any substance is “the number of units of heat required to raise one pound of such substance one degree.” We can explain this farther. When heat is communicated to a body it has two or three functions to perform. Some of it has to overcome the resistance of the air in expanding the body, more of it expands, and the remainder increases the temperature of the body. So some heat disappears as heat, and is turned into energy—“molecular potential energy,”—as it is called, and the rest remains. Of course in objects the molecules vary very much in weight and in their mutual attraction, and the heat requisite to raise equal weights of different substances through the same number of degrees of temperature will vary. This is called capacity for heat, or specific heat. The capacity of different metals for heat can easily be shown. The specific heat of water is very high, because its capacity for heat is great. We can cool a hot iron in very little water, and it takes thirty times as much heat to raise a given weight of water a certain