152 PHYSIOLOGY FOR NURSES 



expansion. If the expansion is in but one direction, it 

 is called linear, if in two, superficial, and in three cubi- 

 cal. Of course expansion really always takes place in 

 three directions. Thus steel rails on a railway expand 

 chiefly in a linear direction, because their bulk in this 

 direction is greatest, but they also expand superficially 

 and cubically. Changes in volume, therefore, result 

 from increase or loss of heat. Advantage is taken of 

 this fact to make the instrument with which the degree 

 of heat is measured the thermometer. Matter which 

 absorbs moisture contracts under the influence of heat, 

 because the rise in temperature expels the moisture. 

 Wood and paper are good examples of such bodies. 



Temperature is not heat, but is simply a measure of 

 the ~hotness of a given body. A degree of temperature is 

 a unit for measuring hotness as a stick of thirty-six 

 inches is a unit for measuring goods. The thermometer 

 is simply a glass tube containing a substance which ex- 

 pands rapidly in the presence of heat or contracts when 

 some of the heat is removed. Mercury and alcohol are 

 the two substances in chief use for this purpose mer- 

 cury because it remains liquid in any but extreme heat 

 or cold, and alcohol because it is practically unfreez- 

 able. 



The scale of a thermometer is arrived at by finding 

 two constant points that of melting ice and that of 

 boiling water. In the centigrade scale the melting point 

 of ice is marked zero, while in the Fahrenheit, thirty- 

 two degrees indicate this point. The boiling point of 

 water is marked 100 in centigrade and 212 in Fahren- 

 heit. One degree Fahrenheit is equal to % degree centi- 

 grade. To convert Fahrenheit to centigrade, therefore, 

 subtract 32 and multiply by %. 



Example: 104 -32 72. 72 x % 360 = 40 C. 



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