/ SOLIDS, LIQUIDS, AND GASES. 381 



But we must not push this mere numerical reasoning 

 too far, seeing that it is quite possible to be continually 

 approaching a given point, without ever reaching it, as 

 when we go on continually halving the remaining distance. 

 The figures in the above do not appear to follow accord- 

 ing to such a law nor, indeed, any other regularity. This 

 probably arises from experimental error, as there are dis- 

 crepancies in the results of different investigators. They 

 all agree, however, 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 

 418-4, at 21=422-9, at 22=427'3, at 23 = 431-4, and at 24 

 atmospheres, their highest experimental limit, 435 '5, thus 

 reducing the rise of temperature between the 23d and 24th 

 atmospheres to 4-1. 



If we could go on heating water in a transparent vessel 

 until this difference became a vanishing quantity, we should 

 probably recognize a visible physical change coincident with 

 this cessation of condensibility by pressure; but this is not 

 possible, as glass would become red-hot and softened, and 

 thus incapable of bearing the great pressure demanded. 

 Besides this, glass is soluble in water at these high tem- 

 peratures. 



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 unmanageable degree of heat. Liquid 

 carbonic acid, which, under a single atmosphere of pressure, 

 boils at 112 below the zero of our thermometer, may thus 

 be raised to a temperature having the same relation to its 

 boiling-point that a red-heat has to that of water, and may 

 be still confined within a glass vessel, provided the walls of 

 the vessel are sufficiently thick to bear the strain of the 

 elastic outstriving pressure. In spite of its brittleness 

 glass is capable of bearing an enormous strain steadily ap- 

 plied, as may be proved by trying to break even a mere 

 thread of glass by direct pull. 



Dr. Andrews thus treated carbonic acid, and the experi- 

 ment, as I have witnessed its repetition, is very curious. 



