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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. 
