x HEAT 139 



expands, and when it has reached the top of the tube, the end 

 is closed by directing a blow-pipe flame against it. This method 

 of closing a tube and keeping the air out is called hermetically 

 sealing it. The thermometer at this stage should be put on one 

 side for some days at least, in order that it may assume its final 

 size, which it does very slowly indeed. The thermometer will 

 now look like one of those in Fig. 57, according to the shape of 

 the bulb which has been blown. 



Graduating the Thermometer Fixed Points. In the 

 graduation of a thermometer the plan always adopted is to 

 choose "two fixed points" from which to number our degrees 

 of temperature. The most convenient lower fixed point we can 

 get is the temperature at which ice melts, or water freezes, for this 

 is always the same if the ice is pure, and remains the same 

 as long as there is any ice left unmelted. The truth of this 

 statement can be verified with a thermometer in the condition 

 shown in Fig. 57. Whenever it is put into melting ice the 

 mercury always stands at the same level, or, as we have seen, 

 melting ice is always at the same temperature and may be used 

 to give one fixed point. The l ' higher fixed point " chosen is 

 that at which pure water boils at the sea-level. We have to 

 make this stipulation, for the boiling point of a liquid is altered 

 when the pressure is changed, being raised if the pressure is 

 greater and lowered if the pressure is less. When the water 

 boils the temperature of the steam is the same as that of the 

 water, and remains so as long as there is any water left. The 

 lower fixed temperature we refer to as the "Freezing Point of 

 Water," the higher as the " Boiling Point." 



Marking the Freezing Point. For this purpose an arrange- 

 ment like that shown in Fig. 58 is veiy suitable. The funnel 

 is filled with pounded ice, which before powdering had been 

 carefully washed ; or snow might, if more convenient, be used. 

 The glass dish catches the water which is formed from the melt- 

 ing of the ice or snow. We make a hole in the pounded ice by 

 thrusting in a pencil or glass tube about the size of the ther- 

 mometer, and into this hole we put the thermometer and 

 support it so that the whole of the mercury is surrounded by the 

 ice or snow. The arrangement is left for about ten or fifteen 

 minutes, until it is quite certain that the tube and mercury 

 are at the same temperature as the melting ice. When this is 

 so the tube is raised until the mercury is just above the ice, 



