THE THERMOMETER. 



same, by dividing ,4^. by 90, which gives ^y^. For each degree 

 of temperature by wftich the mercury is raised, it will therefore 

 receive an increment of volume amounting to the 9990th part of 

 its volume at 32, and it follows, that the weight of mercury which 

 fills the portion, of a thermometric tube representing one degree 

 of temperature, will be the 9990th part of the total weight con- 

 tained in the bulb and tube. 



29. In adopting the dilatation of mercury as a measure of 

 temperature, it is assumed that equal dilatations of this fluid are 

 produced by equal increments of heat. Now, although it is 

 certain that to raise a given quantity of mercury from the freez- 

 ing to the boiling point will always require the same quantity of 

 heat, it does not follow that equal increments of volume will 

 correspond to equal increments of heat throughout the whole 

 extent of the thermometric scale. Thus, although the same 

 quantity of heat must always be imparted to the mercury contained 

 in the tube to raise it from 32 to 212, it may happen that more 

 or less heat may be required to raise it from 32 to 42, than from 

 202 to 212. In other words, the dilatation produced by equal 

 increments of heat, in differents parts of the scale, might be 

 variable. Experiments conducted, however, under all the con- 

 ditions necessary to ensure accurate results, have proved that 

 mercury is uniformly dilated between the freezing and boiling 

 points, or that equal increments of heat imparted to it produce 

 equal increments of volume. 



30. A thermometer having once been carefully graduated may 

 be used as a standard instrument for graduating other ther- 

 mometers, just as good chronometers once accurately set are used 

 as regulators for other time-pieces. To graduate a thermometer 

 by means of such a standard, it is only necessary to expose the 

 two instruments to the same varying temperatures, and to mark 

 upon the blank scale of that which is to be graduated two points 

 corresponding to any two temperatures shown by the standard 

 thermometer, and then to divide the scale accordingly. 



Thus, for example, if the two instruments be immersed in 

 warm water and the column of the standard thermometer be 

 observed to indicate the temperature of 150, let the point at 

 which the mercury stands in the other thermometer be marked 

 150 upon its scale. 



Let the two instruments be then immersed in cold water and 

 let us suppose that the standard thermometer indicates 50. Let 

 the point at which the instrument to be graduated stands be then 

 marked. Let the intervals of the scale between these two points, 

 thus corresponding to the temperatures of 50 and 150, be 

 divided into one hundred equal parts ; each part will be a degree 



156 



