MEASUREMENT OF TEMPERATURE 77 



it usually contains spirits of wine, because this liquid has to be cooled 

 a great deal before it is solidified, that is, made into a solid. But 

 this thermometer cannot be used for any great degree of temperature 

 because spirits of wine is soon changed completely into a vapour when 

 heated to only a comparatively small extent. If it is wished to measure 

 higher temperatures a quicksilver or mercury thermometer is used, 

 because mercury can be warmed a good deal, or, as it is better to say, 

 raised to a high temperature, without being changed into a gas. 



3. The liquid should be in a fine tube of equal bore with a comparatively 

 large bulb at the end. 



Liquids have to be contained in some sort of vessel or else they 

 cannot be kept together. There must be a fine bore, so that the liquid 

 may appear to expand very much for a small change of temperature. 

 The bore must be equal all the way along, that is, the width or 

 diameter of the inside of the tube must be the same all the way 

 along, so that a given amount of expansion in any part of the tube 

 shall mean the same change of temperature ; and, lastly, there must 

 be a large bulb, so that there is a large surface to take the same 

 temperature as that of the substance the temperature of which is 

 being measured. 



Reasons why mercury is used for thermometers. There are 

 many reasons for selecting mercury as the liquid for an ordinary 

 thermometer hi addition to those already mentioned. 



It is a liquid the level of which can be easily seen. 



It does not wet the vessel in which it is contained. 



It expands a considerable amount for a small increment of tem- 

 perature. 



It is a good conductor of heat, and consequently it very quickly 

 assumes the temperature of the body with which it is placed in 

 contact. 



Very little heat is required to raise its temperature, and there is 

 therefore very little loss of heat due to warming the thermometer. 



Construction of a thermometer. Having selected a suitable piece 

 of thermometer tubing, first a bulb must be blown on one end. The 

 glass is melted at this end and allowed to run together and so close up 

 the bore. While the glass is still molten, air is blown down the tube 

 from the other end, the tube being moved round, so that the bulb may 

 be placed symmetrically with reference to it. The bore of the tube is 

 so fine that it is impossible to pour the liquid down it ; some other 

 plan must therefore be adopted. The top of the tube may be enlarged 



