466 PHYSIOLOGICAL PHYSICS. [Chap, xxxvi, 



of the position of the index will be a measure of the 

 differences of temperature experienced. In the same 

 way with a flask of alcohol, provided, again, the pressure 

 be constant, variations in the height of the column of 

 alcohol in the capillary continuation of the neck of the 

 flask will indicate the fact and the amount of varia- 

 tions in temperature. 



Thermometer is the name applied to an instru- 

 ment devised on the principle just laid down for 

 measuring variations and amounts of temperature. If 

 a thermometer be placed in contact with a body 

 warmer than itself, heat will pass from the warmer 

 body till the temperature of both is alike ; the liquid 

 will rise in the thermometer, owing to its expansion, 

 till equilibrium is attained, and the difference between 

 the former and the present level will indicate the 

 difference between the temperatures of the thermo- 

 meter and the body. If the thermometer be placed 

 in contact with a colder body heat will pass from the 

 thermometer till it becomes of the same temperature 

 as the body with which it is in contact ; the liquid 

 will contract in the thermometer, and the diminution 

 of volume will indicate -the decrease of temperature. 



The mercio'ial thermometer is the one in 

 common use. It is made of a capillary tube blown 

 out into a bulb at the lower end. A small funnel 

 is blown at the upper end to permit of filling the 

 tube with mercury. Into the funnel at the upper 

 end a little dry mercury is poured, and the empty 

 bulb is gently heated. Some air is expelled by the 

 heat, and on cooling the remaining air contracts, so 

 that mercury rushes in to fill up the space. The 

 mercury which has thus gained access to the bulb is 

 now heated, and then allowed to cool, when more 

 mercury enter-s. This process is repeated till the 

 bulb is full, and the mercury extends for some 

 distance up the tube. The mercury is then heated to 



