882 SCIENCE IN SHORT CHAPTERS. 



A liquid occupies the lower part of a very strong glass 

 tube, which appears empty above. But this apparent void 

 is occupied by invisible carbonic acid gas, evolved by the 

 previous boiling of the liquid carbonic acid below. We 

 start at a low temperature say 40 Fahr. Then the tem- 

 perature is raised; the liquid boils until it has given off 

 sufficient gas or vapor to exert the full expansive pressure 

 or tension due to that temperature. This pressure stops 

 the boiling, and again the surface of the liquid is be- 

 calmed. 



This is repeated at a higher temperature, and thus con- 

 tinued until we approach nearly to 88 Fahr., when the 

 surface of the liquid loses some of its sharp outline. Then 

 88 is reached, and the boundary between liquid and gas 

 vanishes; liquid and gas have blended into one mysterious 

 intermediate fluid; an indefinite fluctuating something is 

 there filling the whole of the tube an etherealized liquid 

 or a visible gas. Hold a red-hot poker between your eye 

 and the light; you will see an upflowing wavy movement 

 of what appears like liquid air. The appearance of the 

 hybrid fluid in the tube resembles this, but is sensibly 

 denser, and evidently stands between the liquid and gas- 

 eous states of matter, as pitch or treacle stands between 

 solid and li'quid. 



The temperature at which this occurs has been named 

 by Dr. Andrews the "critical temperature"', here the gas- 

 eous and liquid states are "continuous" and it is probable 

 that all other substances capable of existing in both states 

 have their own particular critical temperatures. 



Having thus stated the facts in popular outline, I shall 

 conclude the subject by indulging in some speculations of 

 my own on the philosophy of these general facts or natural 

 laws, and on some of their possible consequences. 



As already stated, the conversion of water . into steam 

 under ordinary atmospheric pressure demands 9G6'6 of 

 heat over and above that which does the Avork of raising 

 the water to 212, or, otherwise stated, as much heat is at 

 work in a given weight of steam at 212, as would raise 

 the same quantity of water to 1178-6 if it remained 

 liquid. 



