44 



HEAT. 



the neck 'of the flask being then closed 

 so as effectually to exclude the air, by 

 firmly tying pieces of bladder over it, 

 or in any other way, the flask must 

 then be suffered to cool until it attains 

 the temperature of the surrounding air, 

 at which time the surface of the fluid 

 will be much lower in the neck of the 

 flask ; a thread being put round to 

 mark its place, and the temperature of 

 the flask having been felt by the hand, 

 a hole may be made with a sharp point 

 through the bladder, so as to admit the 

 pressure of the air. The surface of the 

 fluid will be agitated by the sudden 

 admission of the air, and it is very pro- 

 bable that crystallization will commence 

 at the surface, and proceed rapidly down- 

 wards in a beautiful manner, until the 

 whole becomes solid. If crystallization 

 should not take place on the admission 

 of air, which is sometimes the case, the 



Erocess maybe made to commence by 

 ;tting fall a small solid particle of the 

 same salt into the solution. Sometimes 

 it can scarcely pass below the surface 

 before particles are attracted, which, 

 arranging themselves round the solid 

 nucleus, form beautiful radiating crys- 

 tals that enlarge rapidly, extending 

 throughout the whole mass, which be- 

 comes solid in a very short time. On 

 trying the temperature of the flask with 

 the hand, it will be found quite warm, a 

 considerable quantity of heat being set 

 free in the transition from fluid to solid. 

 A few drops of fluid will probably re- 

 main, the surface of which will stand 

 considerably above the thread on the 

 neck of the flask, proving that the new 

 arrangement of the parts necessaiy to 

 the solid state occupies more room. It 

 has usually been thought that the pres- 

 sure of the air is essential to enable 

 crystallization to take place in this ex- 

 periment ; but sometimes, as has already 

 been stated, the pressure of the air may 

 be admitted without producing the 

 effect, while the introduction of a solid 

 particle generally succeeds. It is more 

 probable, therefore, that when crystalli- 

 zation happens on the admission of air, 

 that it is occasioned by the agency of 

 solid particles admitted along with the 

 air, which serve as nucleii or points of 

 attraction for crystallization to com- 

 mence upon. 



Dr. Black observed appearances at- 

 tending the liquefying of other substances 

 which connected them with the doctrines 

 of latent heat, and he was of opinion 



that the same reasoning which applies 

 to the melting of ice may apply to the 

 melting of all other bodies. 



Dr. Irvine, making some experiments, 

 at the desire of Dr. Black, on sperma- 

 ceti, bees' wax, and some other sub- 

 stances, found that spermaceti absorbed 

 in melting, without becoming sensibly 

 warmer, a quantity of heat sufficient to 

 have rendered the same quantity of 

 melted spermaceti from 141 to 148 

 hotter. Bees' wax, in the same way, 

 absorbed "175; and it appeared to him 

 that tin absorbed as much heat in melt- 

 ing as would raise its temperature in 

 its solid state 500. Dr. Black was of 

 opinion that the softness which takes 

 place in some bodies before they assume 

 the fluid state, depends upon their ab- 

 sorbing a portion of heat, though not 

 in sufficient quantity to produce fluidity. 

 The malleability and ductility of metals 

 were also considered by him as de- 

 pending upon the absorption of heat. 

 " I therefore consider the metals," says 

 he, " as substances which have me 

 power to retain strongly a certain quan- 

 tity of latent heat, which gives them 

 their toughness and malleability ; but I 

 imagine that heat is driven out of them 

 by the violent agitation, compression, 

 and friction of their parts, in hammering 

 them strongly into another shape. Those 

 called the more perfect metals retain 

 this heat with the greatest force, and 

 retain it in some cases, though extended 

 by skilful hammering, to an amazing 

 degree. Tough iron, which is a purer 

 metal than steel, contains more of it 

 than steel does, and shows a little more 

 power to retain it ; from iron it cannot 

 be expelled but by the strokes of the 

 hammer, or violent compression ; from 

 steel it can be separated not only by 

 hammering, but also by sudden and 

 violent refrigeration of the steel from a 

 red-hot state. This happens in the 

 operation called the hardening of steel. 

 The steel is made red-hot in the fire, 

 and then suddenly plunged into cold 

 water. Thus it is made excessively 

 hard, but at the same time perfectly 

 inflexible or brittle. We must, there- 

 fore, conclude that this sudden and 

 violent refrigeration prevents its retain- 

 ing a due portion of latent heat, which 

 it would have retained, had it been 

 allowed to cool slowly and quietly. Iron 

 when heated in the same manner loses 

 but very little of its latent heat." Blacks 

 Lectures, vol. i. p. 140. 



