be decidedly felt and indicated by the thermometer.  Take the case of a liquid suddenly solidifying.  The heat latent in that liquid, and necessary to keep it a liquid, is no longer necessary and comes out, and the substance appears to become hot.  Quicklime is a cold, white, solid substance, but there is a compound of water and lime—­slaked lime—­which is also a solid powdery substance, called by the chemist, hydrate of lime.  The water used to slake the quicklime is a liquid, and it may be ice-cold water, but to form hydrate of lime it must assume a solid form, and hence can and does dispense with its heat of liquefaction in the change of state.  You all know how hot lime becomes on slaking with water.  Of course we have heat of chemical combination here as well as evolution of latent heat.  As another example, we may take a solution of acetate of soda, so strong that it is just on the point of crystallising.  If it crystallises it solidifies, and the liquid consequently gives up its latent heat of liquefaction.  We will make it crystallise, first connecting the tube containing it to another one containing a coloured liquid and closed by a cork carrying a narrow tube dipping into the coloured liquid.  On crystallising, the solution gives off heat, as is shown by the expansion of the air in the corked tube, and the consequent forcing of the coloured liquid up the narrow tube.  Consequently in your works you never dissolve a salt or crystal in water or other liquid without rendering heat latent, or consuming heat; you never allow steam to condense in the steam pipes about the premises without losing vastly more heat than possibly many are aware of.  Let us inquire as to the latent heat of water and of steam.

Latent Heats of Water and Steam.—­If we mix 1 kilogram (about 2 lb.) of ice (of course at zero or 0 deg.  C.) with 1 kilogram of water at 79 deg.  C., and stir well till the ice is melted, i.e. has changed its state from solid to liquid, we find, on putting a thermometer in, the temperature is only 0 deg.  C. This simply means that 79 deg. of heat (centigrade degrees) have become latent, and represent the heat of liquefaction of 1 kilogram of ice.  Had we mixed 1 kilogram of water at 0 deg.  C. with 1 kilogram of water at 79 deg.  C. there would have been no change of state, and the temperature of the mixture might be represented as a distribution of the 79 deg.  C. through the whole mass of the 2 kilograms, and so would be 39-1/2 deg.  C. We say, therefore, the latent heat of water is the heat which is absorbed or rendered latent when a unit of weight, say 1 kilogram of water as ice, melts and liquefies to a unit of water at zero, or it is 79 heat units.  These 79 units of heat would raise 79 units of weight of liquid water through 1 deg.  C., or one unit of liquid water through 79 deg..