EBULLITION. 



311 



panded into a greater one. If the piston had been previously raised, and a 

 small quantity of heat at the same time abstracted from the vapor, a portion of i 

 the vapor would immediately be condensed, and a small portion would be con- j 

 densed by the same loss of heat, in whatever state of compression or rarefac- i 

 tion the steam may exist. This condensation is therefore altogether indepen- ' 

 dent of any effects produced on the density of the steam by any mechanical 

 compression.* 



The pressure on the surface of water, though the principal cause which 

 affects the boiling point, is not the only one. It has been already stated that 

 the material of which the vessel is composed, in which the process of boiling 

 takes place, has also an effect upon the boiling temperature. It is found that 

 in a vessel of glass, water boils at a lower temperature than in a vessel of 

 metal. Foreign matter also held in solution by the water produces a change 

 in its boiling point ; but this should rather be considered as a distinct liquid. 



If heat be applied to other liquids, results will be obtained showing that the 

 phenomena already explained with respect to water, are only instances of a 

 more numerous class, applicable to all liquids whatever. The application of 

 heat to any liquid causes its temperature, in the first instance, to rise ; and 

 this increase of temperature continues until the liquid attains a state similar to 

 that of boiling water, when a thermometer or pyrometer, immersed in it, would 

 become stationary. The continued application of heat now no longer causes 

 the liquid to rise in temperature, but produces vapor rapidly, so that the -liquid 

 boils away in the same manner as already described with respect to water, 

 and all the effects before explained take place, differing only in the tempera- 

 ture at which the ebullition commences, and in the rate at which the vapor is 

 produced. Different liquids attain the stationary temperature of ebullition at 

 different points ; and hence the boiling point becomes a specific character to 

 distinguish material substances. They likewise, in passing into the vaporous 

 form, render different quantities of heat latent. 



Let a thermometer, consisting of two metallic bars, be fixed in a vessel so 

 as to extend across it in a horizontal position, and so that the extremity, bear- 

 ing the graduated scale, shall pass through the side and project outside the 

 vessel. Let melted lead be now poured into this vessel, so as to cover the 

 pyrometric bars, and let the whole be placed on a furnace. The divided scale, 

 during the continued application of the fire, will constantly show an increasing 

 temperature until the lead boils. The expansion of the bars will then cease, 

 and the pyrometer will become fixed in its indication, and will continue fixed 

 until the whole of the lead is evaporated. 



Again, let a common thermometer be immersed in phosphorus at the tem- 

 perature of 300°, and, being placed in a vessel, let it be exposed to the action 

 of heat. It will continue to rise until it attains the temperature of 554°, where 

 it will become stationary, and the phosphorus will boil. The thermometer 

 will become stationary until the whole of the phosphorus is evaporated. 



The correspondence of these results with those obtained in the experiments 

 instituted upon water is obvious. The analogy might be still further confirmed 

 by using a close vessel, like that represented in fig. 1, and carrying over the 

 vapor of the lead, or the phosphorus, into a .vessel exposed to cold, where it 

 might be re-collected in the liquid form. It is clear that, in all these instances, 

 during the process of ebullition, heat has become latent, because heat contin- 

 ues to be supplied to the vaporizing body, although the vapor produced by the 

 supply of such heat is found to have no greater temperature than that of the 

 liquid from which it is produced. The same result would be obtained by simi- 



* I have been the more minute in these details, because my opinions differ from those commonly 

 received respecting the effects of compression upon steam. 



