PRINCIPLES OF THE STEAM-ENGINE. 363 



Careful observation of the instant of change from one 

 of these conditions to another, leads to discoveries of the 

 highest order, and which are keys to the economical ap- 

 preciations of steam-engines. 



Water is not necessarily hotter than any kind of ice ; 

 water may be kept at the temperature of zero without 

 freezing; ice may remain. at zero without melting; but 

 while this water and this ice are both of the same tem- 

 perature, are both at zero, it seems difficult to believe 

 that they do not differ but by their physical properties ; 

 that no element, extraneous to the water so called, distin- 

 guishes the solid water from the fluid. A very simple 

 experiment will clear up this mystery. 



Mix a kilogram of water at zero with a kilogram of 

 water at 79 centigrade ; the two kilograms of the mix- 

 ture will be at a temperature of 39 and a half; that is 

 to say, at the mean of the two constituent fluids. The 

 hot water preserves 39 and a half of its former heat, and 

 has ceded 39 and a half to the cold water ; this is very 

 natural and might have been foreseen. 



But let us repeat this experiment with one modifica- 

 tion : instead of the kilogram of water at zero, let us take 

 a kilogram of ice at the same temperature of zero. From 

 the admixture of this kilogram of ice with the kilogram 

 of water at 79, there will result two kilograms of fluid 

 water, because the ice bathed in the hot water cannot fail 

 to melt and to preserve its former weight ; but do not has- 

 tily attribute to the mixture, as in the preceding instance, 

 a temperature of 39 and a half ; for you would be mis- 

 taken ; the temperature will be only zero ; there will be 

 no trace left of the 79 of heat which the hot water 

 possessed : those 79 disintegrated the molecules of ice 

 they have combined with them, but without heating them 

 at all. 



