HEAT. 



rendered necessary in consequence of 

 an enlarged capacity for heat which the 

 body acquires on its change of state. 

 Ingenious arguments have been ad- 

 vanced in support of both these opi- 

 nions ; and, perhaps, the preponderance 

 of probability may be in favour of the 

 last mentioned. 



CHAPTER X. 



Of Artificial Modes of lowering Tem- 

 perature. 



INTENSE cold may be produced, upon 

 the principle of the rapid absorption of 

 heat by bodies when they pass from the 

 solid to the fluid state. The effects of 

 freezing mixtures depend upon this 

 principle. Solid saline substances are 

 used for this purpose, some of them 

 being much better adapted than 

 others. 



A reduction of 1 7 may be obtained 

 by saturating water at a moderate tem- 

 perature with nitre. Muriate of am- 

 monia lowers the temperature from 26 

 to 28. Nitrate of ammonia dissolved 

 in its own weight of water effects a re- 

 duction from 5U of Fahrenheit's scale, 

 to 4 of the same ; and a mixture of 

 three parts of muriate of lime with two 

 parts of water, lowers the thermometer 

 from 36 to 1 below zero. 



It was discovered by Mr. Walker, 

 that a greater degree of cold may be 

 produced by the solution of several salts 

 at the same time. Five parts muriate 

 of ammonia, five parts nitre, and six- 

 teen parts water, reduce from 50 to 

 11, and the salts maybe again obtained 

 by evaporation, and will answer equally 

 well a great number of times. 



When dilute acid is used instead of 

 water, the solution "is effected more ra- 

 pidly, and the cold produced is there- 

 fore greater. Nine parts of phosphate 

 of soda, six parts nitrate of ammonia, 

 and four parts diluted nitric acid, mixed 

 together, cool the thermometer from 50 

 above zero to 21 below it. 



By the action of acids upon snow 

 great degrees of cold are produced. 

 When the acids are used in an undi- 

 luted state, Lowitz affirms that the 

 muriatic is the most powerful, then the 

 nitrous, and lastly the sulphuric: but 

 when sulphuric acid is diluted, it is 

 more powerful than the others, a de- 



gree of cold equal to 91 having been 

 produced by its agency. 



Mr. Walker, by successive cooling 

 of the materials, succeeded with eight 

 parts of snow and ten parts of diluted 

 sulphuric acid, in reducing temperature 

 to 91 below zero of Fahrenheit. 



The method most generally used de- 

 pends upon the mutual action of snow 

 or ice, and solid salts upon each other. 

 Experiments of this kind were made by 

 Fahrenheit, for the purpose of regu- 

 lating the commencement of his scale. 

 Similar experiments have been made by 

 many others, with greater effect since 

 that time. By mixing solid dry potash 

 with snow, Lowitz obtained a reduction 

 of temperature from 32to 53. With 

 a mixture of three parts dry muriate of 

 lime, and two parts snow, he succeeded 

 in freezing 35 pounds of quicksilver. 



Fourcroy and Vauquelin produced 

 cold, by mixtures of this kind, 65 below 

 zero of Fahrenheit. 



In the course of their experiments, 

 they found that a saturated solution of 

 ammonia in water crystallized at 44 

 and at 56, was converted into a semi- 

 transparent mass, and lost nearly all its 

 pungent odour. At the last-mentioned 

 temperature nitrous acid crystallized, 

 sulphuric ether became thick and milky 

 at 48, and at length formed a white 

 mass composed of small crystals. They 

 failed in all their attempts to solidify 

 alcohol. 



Ammonia in the form of gas, made as 

 dry as possible, was exposed by Guyton 

 de Morveau, in a glass balloon, to a mix- 

 ture of muriate of lime and snow, and 

 was condensed into a liquid at the 

 temperature of 56. 



In forming freezing mixtures in 

 which saline substances are used, it 

 is necessary that they should be newly 

 crystallized, neither humid nor efflo- 

 rescent, and finely pulverized. The 

 materials intended to produce cold 

 ought to be w r ell mixed, and as quickly 

 as possible, in a vessel just large enough 

 to contain it ; and the substance to be 

 congealed should be exposed to the 

 freezing mixture in a thin glass vessel. 

 When snow is used it should be light 

 and dry, and, if possible, newly fallen. 

 Before experiments are made, the ma- 

 terials will frequently require to be 

 -cooled, which is done by exposing them 

 separately in thin glass or tin vessels to 

 freezing mixtures, care being taken not 

 to cool them below the points at which 



