convert them into liquids.  Any force that causes gases to contract will do the same thing, of course—­for example, cold; and ceteris paribus removal of pressure and expansion by heat will act so as to gasify liquids.  When in the expansion of liquids a certain stage or degree is reached, different for different liquids, gas begins to escape so quickly from the liquid that bubbles of vapour are continually formed and escape.  This is called ebullition or boiling.  A certain removal of pressure, or expansion by heat, is necessary to produce this, i.e. to reach the boiling-point of the liquid.  As regards the heat necessary for the boiling of water at the surface of the earth, i.e. under the atmospheric pressure of 15 lb. on the square inch, this is shown on the thermometer of Fahrenheit as 212 deg., and on the simpler centigrade one, as 100 deg., water freezing at 0 deg.  C. But if what I have said is true, when we remove some of the atmospheric pressure, the water should boil with a less heat than will cause the mercury in the thermometer to rise to 100 deg.  C., and if we take off all the pressure, the water ought to boil and freeze at the same time.  This actually happens in the Carre ice-making machine.  The question now arises, “Why does the water freeze in the Carre machine?” All substances require certain amounts of heat to enable them to take and to maintain the liquid state if they are ordinarily solid, and the gaseous state if ordinarily liquid or solid, and the greater the change of state the greater the heat needed.  Moreover, this heat does not make them warm, it is simply absorbed or swallowed up, and becomes latent, and is merely necessary to maintain the new condition assumed.  In the case of the Carre machine, liquid water is, by removal of the atmospheric pressure, coerced, as it were, to take the gaseous form.  But to do so it needs to absorb the requisite amount of heat to aid it in taking that form, and this heat it must take up from all surrounding warm objects.  It absorbs quickly all it can get out of itself as liquid water, out of the glass vessel containing it, and from the surrounding air.  But the process of gasification with ebullition goes on so quickly that the temperature of the water thus robbed of heat quickly falls to 0 deg.  C., and the remaining water freezes.  Thus, then, by pumping out the air from a vessel, i.e. working in a vacuum, we can 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