SOLIDS, LIQUIDS, AXD GASES. 305 



of pressure, only 766-6 more heat would be required, and so on, till 

 we reached a pressure which raised the boiling point to 1178-6 ; the 

 water would then become steam without further heating i.e., the 

 critical point would be reached, and thus, if Watt is right, we can 

 easily determine, theoretically, the critical temperature of water.* 



Mr. Perkins, who made some remarkable experiments upon very 

 high pressure steam many years ago, and exhibited a steam gun at 

 the Adelaide Gallery, stated that red-hot water does not boil ; that if 

 the generator be sufficiently strong to stand a pressure of 60,000 Ibs. 

 load on the safety-valve, the water may be made to exert a pressure 

 of 56,000 Ibs. on the square inch at a cherry-red heat without boiling. 

 He made a number of lather dangerous experiments in thus raising 

 water to a red heat, and his assertion that red-hot water does not 

 boil is curious when viewed in connection with Dr. Andrews' experi- 

 ments. 



I cannot tell how he arrived at this conclusion, having been unable 

 to obtain the original record of his experiments, and only quote the 

 above second-hand. It is worthy of remark that the temperature he 

 names is about 1170, or that which, if Watt is right, must be the 

 critical temperature of the water. Perkins' red-hot water would not 

 boil, being then in the intermediate condition. 



So far, we have a nice little theory, which not only shows how the 

 critical state of water must be reached, but also its precise tempera- 

 ture ; but all this is based on the assumption that Watt made no 

 mistake. 



Unfortunately for the simplicity of this theory, Regnault states 

 that his experiments contradict those of Watt, and prove that the 

 latent heat of steam does not diminish just in the same degree as the 

 boiling-point is raised, but that instead of this the diminution of the 

 latent heat progresses 30 per cent, more slowly than the rise of 

 temperature, so that, instead of the latent heat of steam between 

 boiling-points of 212 and 31'.T falling from 966-6 to 866-6 it would 

 only fall to 895-1 or 69-5 of latent heat for every 100 of tempera- 

 ture. 



If this is correct, the temperature at which the latent heat of steam 

 is reduced to zero is much higher than 1178 -6% and is, in fact, a 

 continually receding quantity never absolutely reached ; but I am 

 not prepared to accept these figures of Begnault as implicitly as is 

 now done in text-books (I was nearly saying " as is now the fashion"), 

 seeing that they are not the actual figures obtained by his experi- 

 ments, but those of his " empirical formulae" based upon them. His 

 actual experimental figures are very irregular ; thus, between steam 

 temperature of 171-6 and 183-2 a difference of 11-6, the experi- 

 mental difference in the latent heat came out as 4-7 ; between steam 

 temperature of 183-2 and 194-8, or 11-6 again, the latent heat 

 difference is tabulated as 8-0. 



Regnault' s experiments were not carried to very high temperatures 

 and pressures, and indicate that as these advance the deviation from 

 Watt's law diminishes, and may finally vanish at about 1500 or 

 1600, where the latent heat would reach zero, and there, according 

 to the above, the critical temperature would be reached. Any addi- 



* Watt's own figure for the latent heat of steam at 212 was 950 = , but I adopt 

 that which is now" generally accepted. 



