HEAT OR CALORIC. 83 



(d.) Freezing water gives out 140 of heat. This warms the in- 

 cumbent air, which rises and affects a delicate thermometer, suspend- 

 ed above the freezing fluid. Freezing is therefore a warming process, 

 and sensibly mitigates the severity of winter ; the 140 being near- 

 }y the whole difference between the extreme climates of the globe, 

 and being given out from the extensive surface of the freezing waters 

 and plants, which are imbued with moisture, it greatly mitigates the 

 atmospheric cold. 



(e.) Melting ice, especially if suspended ', is attended by a descend- 

 ing current of cold air, which is perceptible even to the hand, and 

 still more, by means of a delicate thermometer. Liquefaction is 

 therefore a cooling process, as is perceived also from the chilly air 

 produced by melting snow in a bright day. 



(/*.) Water cooled beloiu 32, if agitated, congeals into a spongy 

 mass of ice; the evolved latent heat raises the temperature to 32, and, 

 a part of the ice slowly melts again. Water may be cooled 20 or 

 more below the freezing point, or 32 of Fahr. This is best done in 

 a tall vessel, with a narrow mouth, and with a film of oil over the 

 surface of the water ; it happens often accidentally in domestic ves- 

 sels, in cold weather. Water thus cooled, immediately commences 

 freezing, if a particle of ice or even a crystal that is floating in the air, 

 happens to enter the fluid. 



(g.) All solids absorb heat when becoming fluid and retain it while 

 in that state. The quantity of heat is different in different cases, 

 and is to be learned only by experiment. 



Sulphur absorbs 143.68 of Fahr. spermaceti 145, lead 162, 

 beeswax 175, zinc 493, tin 500, bismuth 550. Black, Henry. 



(h.) The particular quantity of heat which renders a substance 

 fluid, is called its latent heat, or caloric of fluidity. The word latent 

 was used by Dr. Black, merely to denote the condition in which the 

 heat exists ; latet, it lies concealed. 



It is not a different kind of power, but merely heat in an insensible 

 condition and manifesting its character by a peculiar effect, that of 

 producing fluidity. 



(i.) Freezing mixtures, depend upon these principles. One ingre- 

 dient in them, is always a solid, and in producing the effect of gene- 

 rating cold, this solid always melts or liquefies, and thus absorbs heat. 

 When both substances are solid, as snow and muriate of lime, or 

 snow and caustic potash, or snow and common salt, the effect is of 

 course greater. 



(j.) Heat is evolved during the conversion of fluids into solids. 

 This is well illustrated by the slacking of lime and the mixing of wa- 

 ter with burned plaister of Paris, in both of which cases, the water 

 becomes solid and heat is evolved. 



