304 SCIENCE IK SHORT CHAPTERS. 



eter, 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 applied, as may be proved by trying to break 

 even a mere thread of glass by direct pull. 



Dr. Andrews thus treated carbonic aid, and the experiment, as I 

 have witnessed its repetition, is very curious. 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 temperature 

 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 tempera- 

 ture. This pressure stops the boiling, and again the surface of the 

 liquid is becalmed. 



This is repeated at a higher temperature, and thus continued 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 some- 

 thing 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 

 gaseous states of matter, as pitch or treacle stands between solid and 

 liquid. 



The temperature at which this occurs has been named by Dr. An- 

 drews the " <riti<-al temperature / ' here the gaseous and liquid states 

 are " continuous," and it is probable that all other substances capable 

 of existing in both states have their own particular critical tempera- 

 tures. 



Having thus stated the facts in popular outline, I shall conclude 

 the subject by indulging in some speculations of my own on the phi- 

 losophy of these general facts or natural laws, and on some of their 

 possible consequences. 



As already stated, the conversion of water into steam under ordi- 

 nary atmospheric pressure demands 966-6 of heat over and above 

 that which does the work 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. 



James Watt concluded from his experiments that a given weight of 

 steam, whatever may be its density, or, in other words, under what- 

 ever pressure it may exist, contains the same quantity of heat. Ac- 

 cording to this, if we reduced the pressure sufficiently to bring down 

 the boiling point to 112, instead of 212, the latent heat of the steam 

 thus formed would be 1066-6 instead of 966-6, or if, on the other 

 hand, we placed it under sufficient pressure to raise the boiling point 

 to 312, the latent heat of the steam would be reduced to 866-6 i.e., 

 only 866-6 more would be required to convert the water into steam. 

 If the boiling point were 412, as it is between 19 and 20 atmospheres 



