If a mark is made on the tube, or on a scale fixed to it, at the point which the liquid reaches when the bulb is placed in boiling water; and another mark at the point to which it sinks when the bulb is in melting ice; and the space between the two marks is divided into 180 equal parts, each of these parts is what is called a “degree” in the thermometers ordinarily used in this country (called Fahrenheit). And if the boiling-point is counted as 212° the freezing-point must be 32° (212 - 32 = 180). With the same amount of heat the fluid in the tube always stands at the same degree, and hence the instrument measures =temperature=. That hot water is lighter than cold is easily seen when a bath is filled from two taps, one of hot and one of cold water, which run at the same time. Unless care is taken to stir the water, the top of the bath will be very much hotter than the bottom. Thus, an imperial pint of water weighs a pound and a quarter only at a certain temperature or degree of warmth, namely at 62°; if it is made hotter its volume increases, and therefore its specific gravity diminishes. It was for this reason that in § 22 the weight of the same volume of water was said to be constant =under the same conditions=; and, of course, the same qualification must be borne in mind when we speak of the weight of a cubic inch of water being about 252 and a half grains. Its weight is in fact 252·45 grains only when Fahrenheit’s thermometer stands at 62°—but as this is the temperature of ordinary mild weather, and the expansion or contraction of water for a degree about this temperature amounts to less than 1/3000th of its volume, the weight of a cubic inch may for all practical purposes be taken as 252 and a half grains. 32. =Increase of Heat at length causes Water to become Steam.= Thus a change is effected in the properties of water by heating it ever so little. If it is more strongly heated a still greater change takes place. You know what happens when a saucepan containing water is put on the fire. The water gets hotter and hotter, then it begins to simmer, and finally, when it reaches 212°, it boils away into steam, which passes into the air and disappears. If the boiling is carried on long enough all the water vanishes. It looks at first as if the water had been destroyed by the heat. In reality, however, not a particle of water has been destroyed. It has merely changed its state. The heat has altered it from the state of liquid water into that of gaseous water, =vapour= or =steam=. Try the same experiment with a tea-kettle instead of a saucepan, but only put a little water in the tea-kettle, and shut the lid well down. Then, as soon as the water begins to boil, the steam will shoot out of the spout in a jet; and this will go on as long as any water remains in the kettle.