44 FLUIDITY. 



in the act of dissolving in water, which requires them to become 

 fluid. Nitre, for instance, cools the water in which it is dis- 

 solved 1 5 or 1 8 degrees. A mixture of five parts of sal ammo- 

 niac and five of nitre, both finely powdered, dissolved in nine- 

 teen parts of water, may reduce its temperature from 50 to 10, 

 or considerably below the freezing point of pure water. These 

 salts are necessitated, by their affinity for water, to dissolve when 

 mixed with it and to become fluid, a change which implies the 

 assumption of latent heat. Most of our artificial processes for 

 producing cold are founded upon this disappearance of heat 

 during liquefaction. A very convenient process for freezing a 

 little water, without the use of ice, is to drench finely powdered 

 sulphate of soda with the undiluted hydrochloric acid of the 

 shops. The salt dissolves to a greater extent in this acid than 

 in water, and the temperature may sink from 50 to 0. The 

 vessel in which the mixture is made becomes covered with hoar 

 frost, and water in a tube immersed in the mixture is speedily 

 frozen. 



The same affinity between salts and water may be taken ad- 

 vantage of to cause the liquefaction of ice, as when common salt 

 is strewed upon pavements covered with ice, to melt it. On 

 mixing snow with a third of its weight of salt, the snow is 

 melted, and the temperature sinks nearly to 0. It was in this 

 way that Fahrenheit is supposed to have obtained the zero of 

 his scale. Ices for the table are always made in summer by 

 mixing roughly pounded ice and salt together, and immersing 

 the cream, or other liquid to be frozen, contained in a thin 

 metallic pan, in the cold brine which is produced by the melting 

 of the ice. 



Trie liquefaction of snow by means of the salt, chloride of 

 calcium, occasions a still greater degree of cold. To prepare 

 this salt, marble or chalk is dissolved in hydrochloric acid, and 

 the solution evaporated by a temperature not exceeding 300. 

 It should be stirred, as it becomes dry at this temperature ; and 

 is obtained in a crystalline powder, being the combination of 

 chloride of calcium with two atoms of water. When three parts 

 of this salt are mixed with two of dry snow, the temperature is 

 reduced from 32 to 50. In attempting to freeze mercury 

 by means of this mixture, it is advisable to make use of not 

 less than three or four pounds of the materials. When the 

 materials are divided, and the mercury is first cooled consider- 



