XX.-ON ARTIFICIAL REFRIGERATION. 



By John Gamgee, London, England. 



A.— INTEODUCTIOF. 



Ice, water, and steam, strikingly exemplify the nature of clianges, or 

 the different physical states, produced iu matter by heat. Ice is a trans- 

 parent or translucent solid, which melts into water on the addition of 

 142.G6 heat units (Eegnault). This water has to be raised from the nor- 

 mal temperatiu^e of melting ice, viz, 32° Fahr., to 212° to boil, and 

 every pound converted into steam absorbs 9G5.7 heat units, without 

 afibrding thermometric indications of the change. The heat is said to 

 have become latent, but it is lost in molecular motion. 



The latent heat of water is higher than that of any other agent, and 

 a great dei^ression of temperature ensues when, from a limited amoiuit 

 of water, vapor promiitly rises. Thus, water placed in porous earthen- 

 ware vessels, which are protected from active surface air-currents, by 

 being placed in shallow pits, freezes in Bengal. Windy nights in sum- 

 mer are unfavorable to the process. It is on still and cloudless nights 

 that active radiation, into open space, favors the crystallization of the 

 water. 



Probably with the ice thus formed and the efflorescent salts at hand, 

 the Hindoos first attempted artificial refrigeration. Nitre — the sal petrae 

 of Geber or nitrum of Albertus Magnus — was regarded by the ancients 

 as the primum frigiduinj the cold element of the earth. It occurred on 

 the borders of the Ganges, in Ceylon, and elsewhere as a natural surface 

 deposit, and, after the rainy season, a crust one-third of an inch in thick- 

 ness may be gathered from the ground. It is dissolved out of the salt- 

 petre earths, which at Tirhoot, in Bengal, contain from 8 to 9 per cent, 

 of pure i)otassium nitrate. The mercury descends 18 or 20 degrees if a 

 thermometer be plunged into water simultaneously with nitre. Mixed 

 with ice, a temperature between 5 and 6 degrees below 32° Fahr. is 

 obtained ; and we learn, that as early as 1550, the Eoman nobles cooled 

 their wines by a mixture of snow and nitre. This frigorific mixture is 

 mentioned by Latinus Tancredus, a professor of medicine in Naples, in 

 his work ])e Fama et Siti, published iu 1G07. Yilla Franca, a Spaniard, 

 had been credited with the invention, but in all probability the prac- 

 tice was derived from Asia, and popularized by the Portuguese after the 

 first discovery of India. Common salt and ice were adopted by Fahr- 

 enheit as the means of obtaining the temiierature of his thermom- 



