x HEAT 137 



change of temperature, and we can be sure that if as the result 

 of its contact with any other body, the liquid rises through this 

 amount, that the body with which it is in contact has caused it 

 to experience the change of temperature to which it is equiva- 

 lent, and that the body is at the temperature represented by the 

 higher level of the water. An arrangement of this kind is, 

 then, a "temperature measurer," or, as it is always called, a 

 thermometer. 



It has been found by a long series of experiments that liquids 

 expand much more than solids for a given change of tempera- 

 ture, and that gases again expand still more than liquids. 

 Further, it has been observed that all gases expand to the 

 same amount for a given change of temperature. 



Since gases expand so much more than liquids, "thermometers 

 in which some gas is the form of matter which expands will be 

 very much more accurate than liquid thermometers. Instru- 

 ments are actually made in which air is used. They are called 

 Air Thermometers, and are very accurate indeed. 



Thermometers. Choice of Materials. (1) Liquid. The 

 property of substances which we have now considered is made 

 use of in the construction of temperature measurers or ther- 

 mometers in the following way. First we have to choose a 

 suitable substance, and generally one of two liquids is selected, 

 depending upon the particular purpose to which the thermometer 

 is to be put. If it is to be employed for the measurement of 

 very low temperatures, we shall use alcohol in our thermometer 

 since it is a liquid which freezes with difficulty. If we wish to 

 measure higher temperatures, alcohol would be unsuitable since 

 it boils at a temperature of 78 C. 1 which is, as we shall see, 

 considerably lower than the boiling point of water. For such 

 higher temperatures mercury is used because it does not boil 

 until the temperature of 357 j C. is reached. But though its 

 boiling point is high enough to make it very valuable for the 

 purpose named, it cannot be used for measuring very low tem- 

 peratures, as at - 40 C. it solidifies. 



There are other reasons for selecting mercury in addition to 

 the important one we have just given. It is a liquid whose 

 level can be easily seen ; it does not wet the vessel in which it 

 is contained ; it expands a considerable amount for a small 

 increment of temperature ; it is a good conductor of heat, and 



i These numbers will be understood after the section on the graduation of a 

 thermometer has been read. 



