boil a liquid in such exhausted vessel far below its ordinary boiling temperature in the open air. This fact is of the utmost industrial importance. But touching this question of latent heat, you may ask me for my proof that there is latent heat, and a large amount of it, in a substance that feels perfectly cold. I have told you that a gasified liquid, or a liquefied solid, or most of all a gasified solid, contains such heat, and if reconverted into liquid and solid forms respectively, that heat is evolved, or becomes sensible heat, and then it can 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.