The means employed to find the location of absolute zero are of much interest. It has been observed that when heated a gas tends to expand. If a measured volume of air at 0°C. is cooled or heated 1°C., it changes its volume 1/273, the pressure remaining the same. If it is cooled 10° it loses 10/273, if cooled 100° it loses 100/273 and so on. No matter how far it is cooled the same rate of reduction continues as long as it remains in the gaseous state. From these facts it is concluded that if the cooling could be carried down 273° that the volume would be reduced 273/273 or that the volume of the gas would be reduced to nothing. This is believed to mean that the molecular motion constituting heat would cease rather than that the matter composing the gas would disappear. Scientists have been able to obtain temperatures of extreme cold far down on the absolute scale. Liquid air has a temperature of -292°F., or -180°C. or 93°A. The lowest temperature thus far reported is 1.7°A. or -271.3°C., obtained in 1911, by evaporating liquid helium. =145. The Law of Charles.=--The facts given in the last paragraph mean that if 273 ccm. of a gas at 0°C. or 273° A. are cooled 100°, or to -100°C., or 173°A., then it will lose 100/273 of its volume or have a volume of 173 ccm. If warmed 100°, or up to 100°C., or 373°A., it will have a volume of 373 ccm. It follows then that in every case the volume will correspond to its absolute temperature, providing the pressure remains unchanged. The expression of this fact in scientific language is called the law of _Charles_. _At a constant pressure the volume of a given mass of gas is proportional to its absolute temperature._ Expressed mathematically, we have _V_{1}/V_{2} = T_{1}/T_{2}_. Compare the statement and mathematical expression of the laws of Charles and Boyle. The formulas for the laws of Boyle and Charles are sometimes combined into one expression as follows: _PV/T = P´V´/T´_ or the product of the volume and pressure of a constant mass of gas is proportional to its absolute temperature. Important Topics 1. Heat units; calorie, British thermal unit. 2. Three thermometer scales, fixed points on each. 3. Absolute zero, how determined. Its value on each scale. 4. Law of Charles, its meaning. Combination of laws of Boyle and Charles. Exercises 1. Does ice melt at the same temperature at which water freezes? Express the temperature of freezing water on the three thermometer scales. 2. A comfortable room temperature is 68°F. What is this temperature on the centigrade and absolute scales? 3. Change a temperature of 15°C. to F.; 15°F. to C.; -4°C. to F.; -20°F. to C. 4. The temperature of the human body is 98.6°F. What is this temperature on the absolute and centigrade scales? 5. The temperature of liquid air is -180°C. What is it on the Fahrenheit scale? 6. Mercury is a solid at -40°F. What is this on the centigrade scale?