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- 

 peratui'e, 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 fi'om 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. 



