BOILING POINT, the temperature at which a liquid boils, at the ordinary atmospheric pressure. When a liquid is freely exposed to the air, evaporation goes on constantly from its surface, the heat required to make the change from liquid to gaseous form being absorbed from surrounding bodies. If the liquid is arti ficially warmed, the evaporation goes on at an increased rate; but as its temperature is in creased by the application of heat, there comes a time when mere superficial evaporation does not use up all the heat supplied. Bubbles of vapor then form within the body of the liquid, and the liquid is said to have attained its Moil ing point?' If the supply of heat be further in creased, it is found that the temperature of the liquid remains stationary; bubbles merely form more rapidly, so that the rate of absorption of heat through evaporation is still maintained equal to the rate of supply. The temperature of boiling depends upon the pressure; for at an increased pressure the bubbles are formed in the interior of the liquid with greater difficulty, and therefore not until a higher temperature is attained. The variation from this cause is con siderable. Thus the boiling point of water, under a pressure of one atmosphere, is 212° F., while under twice this pressure, or, as it is commonly stated, under a pressure of two atmospheres, it is about 250° F. At the reduced atmospheric pressure prevailing on the tops of mountains, the boiling point of water is lower than 212° F., and advantage of this fact is taken for determining the heights of mountains by observations of the boiling point at their sum mits. (See HYPSOMETRY). When the liquid is not open freely to the air, but confined in a closed vessel, its temperature can be raised indefinitely by the application of heat, but the vapor in the space above it is denser, and has a greater pressure, at higher temperatures. The correspondence between pressure and tempera ture, under these circumstances, is very exact, although no simple law connecting the two has been formulated. Rankine gave an empiri cal formula expressing the relation between them, of which computers of steam tables have made great use ((Miscellaneous Scientific Papers,' page 1) ; but the physical significance of this formula is unknown. The relation between the pressure and boiling point of a liquid is commonly exhibited by means of a table in which the temperatures of ebullition are set down opposite the corresponding pres sures. The phenomena described above in connection with the free evaporation from a liquid exposed to the air are in general true, but certain qualifications must be made, under certain special conditions. Thus, it is difficult to induce water to boil when it has been freed from dissolved air; and in the entire absence of such air De Luc found that water can be heated as high as 234° F., under ordinary atmospheric
pressure, before boiling, if the experiment is performed with proper care. A liquid thus heated to a temperature in excess of the normal boiling point corresponding to the pressure to which it is subjected is said to be ((superheated? When boiling does finally occur in a superheated liquid, it takes place with almost explosive sud denness, and the loss of vapor is exceedingly rapid until the temperature of the liquid has been reduced by this means to the normal tem perature corresponding to the pressure prevail ing at the time. The temperature at which ebullition takes place is also influenced to a certain extent by the nature of the vessel in which the liquid is heated. Thus Marcet found that in a glass vessel which had been carefully washed out with sulphuric acid, and then well rinsed, pure water does not boil until a tempera ture of 223° F. has been attained. All results of this kind are of an indefinite character, how ever, since they relate to the temperature at which boiling first begins, rather than to the state in which the liquid and its vapor are in a condition of permanent thermal and mechanical equilibrum. Superheated water is in an un stable state, and, according to some authorities, not a few boiler explosions have been due to the superheating of the water present, from some cause, and the subsequent explosive liberation of steam, as the water returned to its normal condition; but this theory as to boiler explosions has never been substantiated by experiment or otherwise. A liquid has a higher boiling point, when it contains some substance in solution, than it has when pure. The effect of dissolving salt or any other electrolyte is complicated by the occurrence of dissociation; but for dilute solu tions of non-electrolytes, like sugar, the follow ing law, first given by Raoult, holds true: If a series of dilute solutions of such substances be prepared, each solution containing, per unit weight of the solvent, an amount of solid pro portional to the molecular weight of the solid, then the solutions so prepared will all boil at the same temperature. (See SOLUTIONS). For marking the "boiler point° upon thermometers, it is the universal practice to expose the ther mometers to the steam rising from the boiling water, rather than to immerse them in the water itself ; for the temperature of the steam is in dependent of the presence of traces of dissolved substances in the water, and also of the action of such accidental or irregular cause as the superheating of the water. See EVAPORATION;