EBULLITION. 315 



art, without falling beyond the limits of the general law, lead to similar 

 conclusions respecting the more numerous class of bodies called permanent 

 gases. 



Bodies existing in the aeriform state are divided into two classes, called 

 vapors and gases. Yapors are those aeriform substances which are known to 

 have been raised from liquids by the application of heat, and which may al- 

 ways be restored to the liquid form by the due abstraction of heat. On the 

 other hand, gases are those aeriform bodies which have never been known to 

 exist in any other than the aeriform state, and which, under all ordinary de- 

 grees of cold, preserve their elastic form. This class includes common air, 

 and a great number of substances known in chemistry under a variety of names, 

 but all comprised under the general denomination of gases. The exact corre- 

 spondence of the mechanical properties of these bodies with those of vapors 

 raised from liquids by heat, naturally leads to the suspicion that they are, in 

 fact, vapors of bodies which vaporize at extremely low temperatures at tem- 

 peratures lower than any which we generally attain even by the processes of 

 art. Such a supposition is perfectly consistent with all the effects which we 

 observe ; for such bodies would then maintain all the gaseous qualities which 

 they are observed to possess at present, though they should be true vapors ca- 

 pable of being condensed, and even solidified, if we possessed practical means 

 of depriving them of a sufficient quantity of the heat which they contain. 



These observations derive considerable probability and force from the results 

 which the improved powers of science have more recently furnished. In pro- 

 portion as more powerful means of extorting heat from gases have been in- 

 vented, a greater number of them have been forced within the limits of the 

 law of condensation. The substance called ammonia was known only as a 

 gas until a temperature of 46 was attained. Exposed to that temperature, it 

 became a liquid. Such a body, in high northern latitudes, would, at different 

 seasons, exist in the different forms of liquid and gas ; in winter it would he 

 liquid, and at other seasons gas. 



Since it is certain that gases may lose a considerable quantity of heat, with- 

 out undergoing any degree of condensation, we must look upon them as vapors ; 

 which, besides the sum of the latent and sensible heat necessary to sustain 

 them in the elastic form, have, subsequently to attaining that form, received a 

 large accession of heat ; and yet, from their nature, with all this supply of 

 heat, their temperature does not exceed the ordinary temperature of the globe. 

 It would be necessary to abstract from them all the heat which they have re- 

 ceived subsequently to taking the vaporous form before condensation could 

 begin. As our power of producing artificial cold is, however, very limited, 

 never having yet exceeded 100 (if, indeed, that limit has been attained), it 

 cannot be surprising that all the redundant heat contained by gases, over and 

 above the sum of latent and sensible heat necessary to maintain them in the 

 elastic form, should not have been extracted by this means. 



Some facility in the attainment of this object may be gained by a knowledge 

 of the fact that the mechanical compression of a gas raises its temperature. 

 If, therefore, a permanent gas be submitted to severe mechanical compression, 

 its temperature will be raised, and the heat which it contains may be more 

 easily withdrawn from it, and imparted to freezing mixtures, or extorted by any 

 of the usual means of exposing it to extremely low temperatures. By contin- 

 ually carrying on the process of compression, additional quantities of heat may 

 be devebped and withdrawn, so that at length we may succeed in reducing 

 the quantity of heat contained in the gas to that sum of latent and sensible 

 heat which seems the limit of the quantity necessary to maintain the elastic 

 form. Any further reduction would be necessarily followed by condensation. 



