communicated will cause part of the water to evaporate without a rise of temperature occurring.

On the other hand, if the temperature of the thermometer falls below 100° C., steam will be condensed on it, and the temperature will be raised by the latent heat rendered up.

In order to provide a larger surface for evaporation, the bulb of the thermometer is often surrounded loosely with cotton wool. Determination of the Boiling Point of a Solution. -As pointed out above, a thermometer when placed in the steam given off from a boiling aqueous solution of a salt, will indicate the boiling point of the water, and not that of the solution. A similar law applies to solutions in general.

In order to determine the boiling point of a solution, the thermometer bulb must be completely immersed. Special precautions must be taken to prevent the occurrence of boiling by bumping, and the consequent rise of temperature of the solution above its boiling point.

Beckmann's Boiling Point Apparatus.-Fig. 85 represents diagrammatically the essential points of Beckmann's apparatus for determining the boiling points of solutions. A test tube A, provided with a side inlet tube B, is used to contain the solution. A piece of platinum wire P, is fused through the bottom of the test tube, and a number of glass beads, G, are also contained by it. The beads and platinum wire serve to promote free ebullition. The bulb of a sensitive thermometer T, similar to that described on p. 15, dips into the solution. A spiral glass tube, K1, serves to condense the vapour given off; the condensed liquid runs back into the solution, so that the strength of the latter is maintained constant.

The test tube containing the solution is surrounded by a glass vessel C, provided with double walls, forming a vapour jacket. A liquid, possessing a boiling point slightly higher than that of the solution in A, is placed in C; some pieces of porous earthenware, D, serve to promote free ebullition, whilst the condensing spiral K2, prevents loss of the liquid.

The whole of the above arrangement is mounted on a stand made from asbestos mill-board. Two pointed Bunsen flames play on the wire gauze E, and the hot gases impinge directly on the lower surface of the vapour jacket C. Direct communication of heat to the vessel, A, is prevented by the double cylinder, F1, F2, of asbestos mill-board, and by a roll, H, of asbestos paper. Thus the solution in A receives heat only from the liquid and vapour in the vessel C.

The solution may be introduced into the vessel A by way of the side

inlet tube B, the condenser K1 being removed.

A Solution of a Non-volatile Substance will possess a higher Boiling Point than that of the pure Solvent. The following table, taken from Lupke's ElectroChemistry, exhibits the elevation of the boiling point produced by dissolving definite quantities of cane sugar in water.

From this table it is evident that the elevation of the boiling point is approximately proportional to the mass of the substance dissolved.

The molecular elevation of the boiling point is obtained by multiplying the elevation, per gram of the dissolved substance, by the molecular weight of the latter. If various substances are used, the molecular elevations will denote the elevations produced by dissolving equal numbers of molecules of the various substances in equal masses of

water.

It has been found that for substances which form electrical non-conducting or badly-conducting solutions in water, the molecular elevation of the boiling point has a constant value of about 5. Thus solutions comprising equai numbers of dissolved molecules in equal masses of water possess the same boiling point.

In the case of aqueous solutions of substances such as sodium chloride, which are good conductors of electricity, the molecular elevation of the boiling point amounts to nearly 10. This is in agreement with the theory, mentioned on p. 170, that such substances are dissociated into simpler elements when dissolved in water.

Variation of the Boiling Point with Pressure.The volume occupied by a given mass of any substance is always much greater in the state of vapour than in that of liquid. In other words, a considerable expansion takes place during the vaporisation of a liquid. Thus I gram of water at 100° C. occupies a volume of 1'043 c.cs. When converted into

steam at 100 C., the volume occupied at normal atmospheric pressure will be about 1650 c.cs.

It is obvious that an increased pressure will tend to hinder this expansion, and therefore to prevent the conversion of water into steam. Hence a higher temperature is required to convert water into steam at a high pressure than would suffice at a low pressure.

EXPT. 51. Take a round-bottomed flask of about 300 c. cs. capacity, half fill it with water, and boil it for a few minutes over a Bunsen flame.

When steam has been given off for a sufficient time for you to feel satisfied that the air in the flask has been expelled, remove the burner, and the instant ebullition ceases, close the mouth of the flask with a well-fitting indiarubber stopper. Invert the flask, and cool it by squeezing cold water on the bottom of it (Fig. 86). You will find that ebullition recommences, and can be produced even when the water has cooled very considerably.

or

In this experiment the pressure of the steam water vapour, at the instant of closing the mouth of the flask, was equal to the atmospheric pressure. On subsequently cooling the bottom of the flask, part of this vapour is condensed, so that the remainder exerts a pressure less than that of the atmosphere. Ebullition recommences because less hindrance is given to the formation of vapour.

As this point is one of some importance, the following explanation, couched in terms of the kinetic theory, should be carefully followed.

At the instant when the flask is closed, the number of molecules returning into the liquid in a given time is just equal to the number leaving its surface in that time. On cooling the bottom of the flask, the total number of molecules in the state of vapour is enormously de

creased, so that fewer have an opportunity of re-entering the liquid. Consequently the number of molecules leaving the liquid is greatly in excess of the number returning to it, thus giving rise to the appearance of ebullition.

In the manufacture of sugar, the aqueous solutions are concentrated by boiling under diminished pressure, the necessary temperature being thus reduced, and charring of the sugar avoided.

A table giving the boiling point of water for various pressures is given on p. 27. Additional information will be found in Chap. XVI.

The Hypsometer. During the ascent of a mountain the limits of the atmosphere are continually approached as greater and greater altitudes are reached. The atmospheric pressure at any place is equal to the weight of the air contained in an imaginary tube of unit sectional area, extending vertically from the place in question to the furthest limits of the atmosphere. Hence it is evident that the barometric pressure falls as we ascend to greater altitudes.

When the pressure at any stage of a mountain ascent has been obtained, the height of the station above the sea level may be calculated, or obtained by reference to tables.

A barometer may be used to determine the required pressure. But as mercury barometers are somewhat cumbersome, and other barometers are not always to be depended upon, it is usual to employ other means for determining the pressure.

The temperature at which water boils is, as we have seen, related to the pressure to which its vapour is subjected. Hence an observation of the boiling point of water may be used to determine the barometric pressure, and from that the height of the station above the sea level.

The instrument used for determining the boiling point of water, in order to determine the altitude of a station, is called a Hypsometer. Fig. 87 shows such an instrument (made by Messrs. Yeates and Sons of Dublin), as a whole and in parts.

Water is contained in a metal vessel C. This is provided with bent metal tubes D, communicating with its interior. A lamp A is screened from draughts by a case B; the flame plays on the tubes just mentioned, The hot gases generated by combustion escape into the open air through oblique tubes E which pass through the water.