SOLIDS, LIQUIDS, AKD GASES. 303 



It may be seen from the above that, with the exception of one 

 irregularity, there is a continual diminution of the additional tem- 

 perature which is required to overcome an additional atmosphere of 

 pressure, and if this goes on as the pressure and temperatures ad- 

 vance, we may ultimately reach a curious condition a temperature 

 at which additional pressure will demand no additional temperature 

 to maintain the gaseous state ; or, in other words, a temperature may 

 be reached at which no amount of pressure can condense steam into 

 water, or at which the gaseous and liquid states merge or become 

 indifferent. 



But we must not push this mere numerical reasoning too far, see- 

 ing that it is quite possible to be continually approaching a given 

 point, without ever reaching it, as when we go on continually halv- 

 ing the remaining distance. The figures in the above do not appear 

 to follow according to such a law nor, indeed, any other regularity. 

 This probably arises from experimental error, as there are discrepan- 

 cies in the results of different investigators. They all agree, how- 

 ever, in the broad fact of the gradation above stated. Dulong and 

 Arago, who directed the experiments of the French Government 

 Commission for investigating this subject, state the pressure at 20 

 atmospheres to be 4184, at 21 - 422-9, at 22 = 427-3, at 13 = 431-4, 

 and at 24 atmospheres, their highest experimental limit, 435-5, thus re- 

 ducing the rise of temperature between the 23d and 24th atmosphere 

 to 4-1. 



If we could go on heating water in a transparent vessel until this 

 difference became a vanishing quantity, we should probably recog- 

 nize a visible physical change coincident with this cessation of con- 

 densibility by pressure ; but this is not possible, as glass would be- 

 come red-hot and softened, and thus incapable of bearing the great 

 Pressure demanded. Besides this, glass is soluble in water at these 

 igh temperatures. 



If, however, we can find some liquid with a lower boiling-point, we 

 may go on piling atmosphere upon atmosphere of elastic expansive 

 pressure, as the temperature is raised, without reaching an unman- 

 ageable degree of heat. Liquid carbonic acid, which, under a single 

 atmosphere of pressure, boils at 112 below the zero of our thermom- 



