138 ELEMENTARY GENERAL SCIENCE CHAP. 



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. 



(2) The Tiibe. Liquids, as we know, must be contained in 

 vessels to keep them together. Consequently, the liquid we 

 have chosen will have to be enclosed in some sort of case. For 

 this purpose a glass tube is used, and some care must be exer- 

 cised in selecting one. It should have a narrow bore, so that a 

 given expansion of the liquid shall be spread over a considerable 

 distance. The bore must be of equal size, or as nearly as we 

 can get it, in every part, and as it is impossible to get one which 

 is quite regular it is usual for very accurate work to calibrate 

 it, i.e., to find the volume of each part of the bore. 



Construction of a Thermometer. Having selected a suitable 

 piece of thermometer tubing, a bulb must be first blown 011 

 one end. The glass is melted at this end and allowed to run 

 together and so close up the bore, and while the glass is still 

 molten, air is blown down the tube from the other end, keeping 



FIG. 57. Thermometers before Graduation. 



the tube moved round, so that the bulb is symmetrically placed 

 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 tube is warmed and inverted in 

 some of the liquid. Let us suppose we are using mercury. 

 Warming the tube makes the air inside it expand, and of 

 course some is driven out. As the tube cools the mercury is 

 forced in by the weight of the atmosphere to fill the place of the 

 expelled air. By repeating this alternate process of warming 

 and cooling, under the circumstances we have described, enough 

 mercury is soon introduced into the tube. The next step is 

 to seal up the tube, leaving no air above the mercury ; to 

 do this the bulb is heated to a temperature slightly higher 

 than we shall want our thermometer to register, the mercury 



