containing some ether. Three holes are bored in its cork; through one passes the tube a, which reaches nearly to the bottom of T; through the other passes a short tube, b, and through the third passes a thermometer, c. Another thermometer, d, shows the temperature of the air. Airis forced through a or drawn through b; in both cases it bubbles through the ether in T and evaporates it, thereby depressing its temperature, and consequently that of the tube T;

d

Fig. 31.

T

after some time the surface of T is seen to become dull, owing to the condensation on its surface of the water of the air. The temperature of the ether is then read on the thermometer c, and that of the air on the thermometer d. The difference measures the degree of moisture in the air-the greater the difference the less being the moisture, or the dryer the air.

CHAPTER XII.

EBULLITION.

§ 129. The tension of the vapour of a liquid increases with the temperature. If the temperature be so raised that the vapourtension is exactly equal to the pressure of the air, the two are in unstable equilibrium, and the slightest increase of temperature will cause the liquid to boil. Bubbles of vapour are formed in the liquid; these rise and break on the surface, whence they spread through the air. The temperature at which this ebullition or boiling takes place is called the "boiling-point" of the liquid.

§ 130. The boiling-point of a liquid is of course found by examining a thermometer whose bulb is either in the liquid as it boils, or in the vapour thence arising.

§ 131. The boiling-points of different liquids under the same circumstances are very different. The following list shows the

temperature at which a few liquids boil under the ordinary pressure of the air (760 millims.).

§ 132. The boiling-point of a liquid varies slightly with the nature of the surface of the vessel which contains it. Liquids boil at the lowest temperatures (and such are regarded as their true boiling-points) in metallic vessels and such as have rough surfaces. In very smooth glass vessels the liquid frequently rises in temperature a few degrees above its true boiling-point; then takes place a sudden burst of vapour, which carries off the extra heat as latent heat, the residual liquid sinking in temperature.

§ 133. Difference of boiling-point often offers a convenient means for the separation of liquids from a mixture. The manufacture of rum, whisky, or spirits of wine from fermented liquids depends upon the fact that alcohol boils at a lower temperature than water, and that, if a mixture of the two be heated to a temperature between the boiling-points of the two (78° C. and 100° C.), most of the alcohol is distilled off and most of the water remains behind.

§ 134. The vapour of a boiling liquid is generally condensed by being made to pass into a vessel externally cooled by water (fig. 32).

Fig. 32.

§ 135. When the pressure upon a liquid is increased, the temperature at which it boils is raised. The heat must be greater before the tension of the vapour overcomes the increased pressure. On the tops of mountains the pressure of the air is less, because there is a less quantity of air above, and that of a less density. Hence in high regions water boils below 100° C. At the bottom of mines it boils above 100° C. By determining the temperature at which water boils, we may therefore get a measure of the height of a mountain.

§ 136. Also, if the pressure of the air be lessened artificially by its total or partial removal, the boiling-points of liquids at the surface of the earth are lowered.

Use is made of this in the arts-as, for instance, in evaporating the water from cane-juice or in sugar-refining. If the saccharine liquid be boiled and concentrated under the ordinary atmospheric pressure, the heat necessary to boil it is so great as to blacken the sugar; but by boiling it in vacuo the boiling-point is lowered, and the syrup may be so concentrated, without injury to its colour, that it crystallizes on cooling.

§ 137. If a flask of water be boiled briskly (so as to be full of only water and steam) and then corked, removed from the flame and inverted, the water may be made to boil long after its temperature has fallen below 100° C., by pouring cold water on the upper part; for on doing so the steam is condensed, and, a partial vacuum being formed, the water boils

at a lower temperature.

§ 138. The relation between the boilingpoint and pressure is seen on heating a quantity of water, W (fig. 33), in a boiler, B, through the top of which passes, steam-tight, a tube, T, open at both ends and dipping below into some mercury beneath the water. A thermometer, A, also passes through the side of the boiler. When the boiler is heated the steam presses upon the surface of the water and forces the mercury up the tube T. The

Fig. 33.

T

B

A

W

higher the temperature, as shown by A, the greater the pressure, as shown by the level of the mercury in T. The following Table shows the temperatures at which water boils when subjected to the pressure of more than one atmosphere :—

The pressure of steam in a boiler may thus be known by finding the temperature of the steam.

§ 139. The boiling-point of a liquid is also altered by dissolving in it solids or other liquids. The dissolved body, if it be a solid, appears to adhere to the liquid and keep back the vapour, necessitating the application of a greater heat. Thus a saturated solution of common salt boils at 108°3 C.

§ 140. The following diagram illustrates, and may serve as a means of recalling, the relation of heat to the three forms of matter.

CHAPTER XIII.

CONDUCTION OF HEAT.

§ 141. When one part of a body receives a certain addition to its temperature, the heat travels through the mass of the body in all directions, until the entire body attains a higher temperature. If, for instance, we suppose a mass of iron of 100 cubic inches to have the uniform temperature of 0° C., then, if one of those cubic inches be suddenly heated to 100° C., it will heat the remaining 99 cubic inches to 1o C., itself falling in temperature to 1o C., so that the result will be 100 cubic inches at 1° C.

§ 142. The passage of heat through the mass of the substance, without any sensible motion taking place in the matter itself, is called the conduction of heat; and the substance through which the heat travels is called a conductor.

§ 143. Further, if any part of a body receives a continual supply of heat by being constantly maintained at the same elevated temperature (as by plunging the part into boiling water), the whole of the body will gradually acquire the same temperature by conduction.

§ 144. Inversely, if the temperature of one part of a body is reduced, the heat from the warmer is conducted into the cooler part until the whole body has a uniform temperature. Thus, if a mass of iron of 100 cubic inches volume have the temperature 100° C., and if one of those cubic inches be cooled to 0° C., the heat will pass into the cool cubic inch until its temperature reaches 99° C. The remaining 99 cubic inches having each thereby lost 1° C., they also will have the temperature 99° C., so that the entire mass will have the temperature of 99° C.

§ 145. Again, if heat be continually withdrawn from a part of a body (as by plunging that part into melting ice), heat will travel through the rest of the body towards the cooler part, until the whole body attains the temperature of the cooler part.

§ 146. Conduction of heat through a body is a particular instance