134 



THE THERMOMETER. 



other, then that part where it occupies a less extent must be greater in diam- 

 eter than other parts, and the bore is consequently not uniform. 



For ordinary domestic purposes, and even for most scientific observations, 

 thermometer tubes can be easily obtained of sufficiently uniform bore ; but in 

 scientific experiments, where the utmost possible accuracy is sought, it has 

 been thought better not to depend on the uniformity of the bore, but to graduate 

 the scale independently of this condition. Such a graduation may be effected 

 by causing a drop of mercury to move from end to end of the tube, and en- 

 graving on the glass with a diamond a number of divisions regulated by the 

 space which the drop of mercury occupied in different parts of the bore. These 

 divisions, whether equal or unequal, would evidently contain the same quantity 

 of mercury, and correspond to equal dilatations or contractions of the fluid.* 



Let us suppose, then, that a tube has been obtained of uniform bore, and a 

 bulb blown upon its extremity, and that we are furnished with pure mercury. 

 The next object is to fill the tube with the mercury. If the tube had not been 

 capillary, but had a bore of considerable magnitude, the mercury could have 

 been easily introduced by pouring it through the tube into the bulb ; but the 

 bores of tubes commonly used for thermometers are much too small to admit 

 of this process. A method of filling the tube is practised which depends part- 

 ly on the high expansibility of atmospheric air, and partly on the atmospheric 

 pressure. The bulb of the tube is held for some time over the flame of a spirit- 

 lamp, so that the air contained in it becomes intensely heated. This air, there- 

 fore, expands, and becomes highly rarefied, so that the quantity or weight of 

 air contained in the bulb and tube at length bears a very inconsiderable propor- 

 tion to that which was contained in it at the ordinary temperature of the atmo- 

 sphere. At the same time, another purpose is answered by this process. A 

 thin film of moisture, attracted from the atmosphere, or in the process of blow- 

 ing the bulb, is liable to attach itself to the interior surface of the bulb and bore ; 

 and if this film were allowed to remain on the tube, it would disturb the indi- 

 cations of the instrument, by becoming mixed with the mercury, and expanding 

 with it in different degrees, so that the apparent expansion would be partly de- 

 pendant on the expansion of the mercury, and partly on the expansion of the 

 vapor arising from this film of moisture. By the process of heating the bulb, and 

 rarefying the air contained in the tube, this film of moisture is effectually evapo- 

 rated and expelled, and nothing remains in the tube but a very small quantity 

 of highly-rarefied air. In this state the tube is inverted, placing the bulb up- 

 ward, and the open end of the tube is plunged in a vessel containing pure mer- 

 cury. The heat by which the air contained in the bulb was rarefied being now 

 removed, the air begins to resume its former temperature, and all communica- 

 tion with the atmosphere being thus cut off by the open end of the tube being 

 immersed in the mercury, no supply of air is admitted to fill the space caused 

 by the contraction of the air remaining in the tube. Meanwhile, the pressure 

 of the atmosphere acts on the surface of the mercury in the cistern, and presses 

 it up in the tube in the same manner, and from the same cause by which mer- 

 cury is sustained in the barometer. In this manner the mercury will be found 

 to rise in the thermometer tube, and ultimately to pass into the bulb, the greater 

 part of which will be filled. The small quantity of rarefied air, now contracted 

 into very limited dimensions, will occupy the upper part of the bulb. Let the 

 tube be now once more inverted, placing the open end upward, and let the bulb 

 containing the mercury be again held over the flame of a lamp. After some 

 time, the bubble of air which remains intermixed with the mercury will be forced 

 out of the tube by the expansion caused by the heat. The bulb must still be 



* This method of graduation was practised by Gay-Lussac. 



