[26] The cooling effect produced in the solution of solids (and also in the expansion of gases and in evaporation) is applied to the _production of low temperatures_. Ammonium nitrate is very often used for this purpose; in dissolving in water it absorbs 77 units of heat per each part by weight. On evaporating the solution thus formed, the solid salt is re-obtained. The application of the various _freezing mixtures_ is based on the same principle. Snow or broken ice frequently enters into the composition of these _mixtures_, advantage being taken of its latent heat of fusion in order to obtain the lowest possible temperature (without altering the pressure or employing heat, as in other methods of obtaining a low temperature). For laboratory work recourse is most often had to a mixture of three parts of snow and one part of common salt, which causes the temperature to fall from 0° to -21° C. Potassium thiocyanate, KCNS, mixed with water (3/4 by weight of the salt) gives a still lower temperature. By mixing ten parts of crystallised calcium chloride, CaCl_{2},6H_{2}O, with seven parts of snow, the temperature may even fall from 0° to -55°. [27] The heat which is evolved in solution, or even in the dilution of solutions, is also sometimes made use of in practice. Thus caustic soda (NaHO), in dissolving or on the addition of water to a strong solution of it, evolves so much heat that it can replace fuel. In a steam boiler, which has been previously heated to the boiling point, another boiler is placed containing caustic soda, and the exhaust steam is made to pass through the latter; the formation of steam then goes on for a somewhat long period of time without any other heating. Norton makes use of this for smokeless street locomotives. [28] [Illustration: FIG. 17.--Curves expressing the contraction, quantity of heat, and rises of temperature produced by mixing sulphuric acid with water. Percentage of H_{2}SO_{4} is given along the axis of abscissae.]