312 



EBULLITION. 



lar experiments made on other substances ; and we may, therefore, generalize 

 the facts established by the experiments already described upon water, and 

 state that all bodies, when in the liquid form, are capable, by increasing their 

 temperatures, of being converted into vapor ; and that in this conversion a large 

 quantity of heat must be supplied, which becomes latent in the vapor, because, 

 notwithstanding the increased supply of heat given to it, it exhibits no corre- 

 sponding increase of temperature. 



There is no liquid upon which the effects of heat have been so minutely ex- 

 amined as water. The latent heats of a few other liquids have been accurately 

 determined ; but much still remains to be done in this department of physics. 

 Count Rumford examined the latent heats of several vapors, by causing them 

 to be condensed in a refrigeratory, so that they imparted their latent heat 

 to water. He then determined the weight of the liquid which had been con- 

 densed, and, by comparing with it the heat imparted to the water in the re- 

 frigeratory, he obtained the latent heat. Dr. Ure and M. Despretz also made 

 experiments on some liquids, the results of which were as follows : 



The boiling points of all liquids are affected by pressure in the same man- 

 ner as the boiling point of water, every increase of pressure causing it to fall. 

 In comparing the boiling points of different liquids one with the other, it is, 

 therefore, necessary to take them all under the same pressure ; and the pres- 

 sure usually adopted for this purpose is the medium pressure of the atmo- 

 sphere, or thirty inches of mercury. 



The comparison of the melting and boiling points of bodies does not present 

 any general feature which could serve as a basis for any obvious inference, 

 connecting the phenomena of fusion and ebullition with their other properties. 

 Generally, but not invariably, the higher on the scale of temperature the melt- 

 ing point is, the higher will be the boiling point ; but to this there are many 

 exceptions. Mercury freezes at 39 below 0, and boils at a temperature of 

 about 660; while, on the other hand, phosphorus melts at 140 above the 

 melting temperature of mercury, and boils at about 110 below the boiling 

 temperature of that metal. 



Since, by continually imparting heat to it, a body in the liquid state at length 

 passes into the form of vapor or air, analogy would lead us to expect that, by 

 continually withdrawing heat, a body in the aeriform state would at length re- 

 turn to the liquid state. In the case of vapor raised from liquids by heat, this 

 is found to be universally true. In the experiment illustrated by figure 1, the 

 steam of water, having passed from the heated vessel to one maintained at a 

 lower temperature, was caused to impart its heat to the surrounding medium, 

 and immediately returned to the liquid state. The same result would be ob- 

 tained under the same circumstances in any liquid body vaporized. The vapor, 

 being exposed to cold, is deprived of a part of that heat which is necessary to 

 sustain it in the aeriform state, and a portion of it is accordingly restored to 

 the liquid form, and this continues until, by the constant abstraction of heat, 

 the whole of the vapor becomes liquid. As a liquid, in passing to the vapor- 



