THE THERMOMETER. 203 



be now transferred to a vessel of boiling water at the time when the 

 barometer stands at the altitude of 30 inches : the mercurv in the tube 

 will be observed to rise until it attain a certain elevation, and will there 

 maintain its position. It will be found, that though the water continue 

 to boil, the mercury in the tube will not continue to rise, but will main- 

 tain a fixed position : let the point to which the mercury nas risen in 

 this case, be likewise marked upon the tube. 



The two points thus determined, are called the freezing and the 

 Boiling points. If the distance upon the tube between these two points be 

 divided into one hundred and eighty equal parts, each of these parts is 

 called a Degree ; and if this division be continued, by taking equal divi- 

 sions below the freezing point, until thirty-two divisions be taken, the last 

 division is called the Zero, or nought of the thermometer. It is the point 

 to which the mercury would fall if the thermometer were immersed in a 

 certain mixture of snow and salt When thermometers were first inven- 

 ted, this point was taken as the zero point, from an erroneous supposition 

 that the temperature of such a mixture was the lowest possible 

 temperature. 



The degrees upon the instrument thus divided are counted upwards 

 from the zero, and are expressed, like the degrees of a circle, by placing 

 a small () over the number. Thus it will be perceived that the 

 freezing point is thirty-two degrees of our thermometer, and the boiling 

 point will be found by adding one hundred and eighty to thirty-two 

 degrees ; it is therefore two hundred and twelve degrees. 



The temperature of a body, then, is that elevation to which the thermo- 

 meter would rise when completely immersed in the component matter of 

 the body. Thus, if we should immerse the thermometer, and should find 

 that the mercury would rise to the division marked one hundred degrees, 

 we should then affirm that the temperature of the water was one hundred 

 degrees. 



The dilation which attends an increase of temperature is one of the 

 most universal effects of heat. It varies, however, in different bodies ; 

 it is least in solid bodies ; greater in liquids ; and greatest of all in bodies 

 in the aeriform state. Again, different solids are differently susceptible 

 of this expansion. Metals are the most susceptible of it; but metals of 

 different kinds are differently expansible. 



As an increase of temperature causes an increase of bulk, so a diminu- 

 tion of temperature. causes a corresponding diminution of bulk ; and the 

 same body always has the same bulk at the same temperature. 



