14 THE THERMOMETER. 



not wider at one place than another. Tubes of very narrow 

 bore, and which are called capillary., the bore being like a hair 

 in magnitude, are now alone employed. Such tubes are made 

 by rapidly drawing out a hollow mass of glass while soft and 

 ductile under the influence of heat. The central cavity still 

 continues, becoming the bore of the tube, and would not cease 

 to exist although the tube were drawn out into the finest thread. 

 From the mode in which capillary tubes are made, their 

 equality of bore and suitableness for thermometers, cannot al- 

 ways be depended upon. The bore is frequently conical, or 

 wider at one end than at the other. It is tested by drawing up 

 into the tube a little mercury, as much as fills a few lines of the 

 cavity. The little column is then moved progressively along 

 the tube, and its length accurately measured, at every stage, by 

 a pair of compasses. The column will measure the same in 

 every part of the tube, provided the bore does not alter. Not 

 more than one-sixth part of the tubes made are found to pos- 

 sess this requisite. 



Satisfied with the regularity of the bore, the thermometer- 

 maker softens one extremity of the tube, and blows a ball upon 

 it. This is not done by the mouth, which would moisten the 

 interior, by introducing watery vapour, but by means of an 

 elastic bag of caoutchouc, which is fitted to the open end of the 

 tube. He then marks off the length which the thermometer 

 ought to have, and above that point expands the tube into a 

 second bulb a little larger than the first. It has then the form 

 of Figure 3. After cooling, the open extremity of the tube is 

 FIG. 3. plunged into distilled 



and well-boiled mercu- 

 ry, and one of the bulbs 

 heated so as to expel 

 air from it. During the 

 cooling, the mercury is 



sfl^ drawn up and rises in- 



^ to the ball a. It is made 



to pass from thence into the ball b, by turning the instrument, 

 so that b is undermost, and then expelling the air from that 

 bulb by applying heat to it, after which the mercury descends 

 from the effect of cooling. The ball b, being entirely filled 

 with mercury, and a portion left in , the tube is supported by 

 an iron wire, as represented in the figure, over a charcoal fire, 



O 



