PRINCIPLES OF THE JIECIIANICAL THEORY OF HEAT. 271 



•which is wrapped around with wire and provided with a hrass appendage which 



contains a duct, ch>sed at will by a cock, wc introduce a little water, so that 



the glass may be moistened within by a few drops. The balloon is then screwed to 



a compressing pump, and so much air is pumped pig, 12. 



in that the tension in the interior of the balloon 



shall amount to about three atmospheres. 11" the 



cock be now closed, and the balloon be unscrewed 



and laid in some warm place, whether in the 



vicinity of a stove or in the sunshine, the vapors 



in the interior will, after a while, have acquired 



the elasticity corresponding to the temperature, 



and appear perfectly transparent. If the cock bo 



now opened, air and vapor will rapidly escape 



from the balloon, and the latter will l)e tilled with 



a. thick mist. 



In a somewhat altered form, the experiment ad- 

 mits of being executed as follows : Let the bell of 



an air-pump be sprinkled on the inside with a 



little water, and after being placed on the plate 



of the pump, be left some time in a rather warm 



chamber. As soon now as we begin to discharge 



the air, a white mist will be formed within the 



bell. 



A corresponding experiment has been instituted 

 by Hirn on a large scale. A straight copper cylinder, two metres long and 15 

 centimetres in diameter, v/as closed at both ends by flat plates, in the middle 

 of w-hich were openings two centimetres wide and closed by plates of glass 

 cemented therein. This cylinder was on one side placed in connection with a 

 steam-boiler, while on the other it bore a discharge tube which was furnished 

 with a wide cock. Into the cylinder was now introduced steam of a higli tension, 

 while the escape-cock was only "partially opened, so that all the air might find 

 an issue. The highly condensed vapor, (five atmospheres, for example,) which 

 in this way fills the cylinder, is now perfectly transparent, so that all objects 

 are plainly visible through the glass plates mentioned above. If the influx of 

 steam froui the boiler be afterwards wholly shut ofl", and the escape-cock be 

 suddenly and fully opened, so that the steam promptly expands to a tension of 

 one atmosphere, a mist of such density is formed in the cylinder that its contents 

 appear completely opaque. 



IX.^TIIE MELTING OF ICE. 



Aft(,'r having applied the principles of the mechanical theory of heat to the form- 

 ation of steam, we will })roceed to consider them in relation to the phenouiena 

 of melting, and shall here treat exclusively of the melting of ice. 



Let us suppose one kilogram of ice at 0° C. to be contained in a vessel, under 

 the pressure of one atmosphere. If heat be communicated to this ice, ivhUc llic 

 ivcsaure remains unaltered, the ice passes gradually into water, but the tenqiera- 

 tnre continues at 0° until the liipiefaction is wholly completed. From this 

 moment first begins the elevation of temperature, if the supply of heat is main- 

 tained. 



The quantity of heat which is necessarv thus, under the pressure of one atmos- 

 phere, to convert one kilogram of ice at'o° into water at 0°, taking the mean 

 of the best experiments, is, r==79.03r, units of heat. This (piantity is usually 

 called, as in the case of steam, the latent heat. 



The i)henoinen()n of melting is wholly analogous to that of evaporation. A 

 part of the heat supplied to the ice is expended in overcoming the cohesion, the 

 other part in external work, as, during the melting, a change of volume takes 

 place. 



