12 



THERMOMETER AND PYROMETER. 



kept under the surface of the mercury, 

 and the instrument to be retained as 

 nearly in the horizontal position as pos- 

 sible, while the flame of a newly snuffed 

 candle is applied to the bulb, so as to 

 boil the included mercury. Thus the 

 remaining air will be expelled ; and on 

 removing the candle, the mercury will 

 suddenly fill the ball and part of the 

 tube. 



4. To ensure a delicate thermometer, 

 the mercury is next to be boiled in the 

 thermometer. For this purpose a slip 

 of clean writing paper is to be rolled 

 tightly around the upper part of the 

 tube, so as to form, beyond the orifice, 

 a cup or cylinder capable of containing 

 as much mercury as the bulb : secure 

 this round the tube with a thread, put 

 a drop of mercury into the paper cavity, 

 and again apply heat to the bulb, hold- 

 ing the tube by the part covered by the 

 paper. The mercury will soon boil, and 

 about one -half of the contents of the 

 ball will rush up into the paper cup. 

 On removing the bulb from the candle, 

 the mercury will suddenly return. Re- 

 peat this operation again and again, 

 until the speedy boiling of the mercury, 

 and the diminished noise and agitation, 

 show that the whole has been well 

 heated, and air and moisture expelled 

 from it. 



Should there be the least moisture in 

 the tube before this part of the opera- 

 tion, it is very likely to burst the bulb ; 

 and the same accident is likely to hap- 

 pen, if the mercury be too strongly 

 boiled the first or second time. 



An experienced eye will readily judge 

 what range of scale the thermometer 

 will have ; but this point can easily be 

 ascertained, before the tube is closed, 

 by heating the bulb in the mouth, and 

 then immersing it in cold water or 

 melting ice. When the latter is used, 

 the operator can at pleasure fix how 

 far from the bulb he will have the 

 freezing point ; for, by keeping the tube 

 more or less filled, he can adjust that 

 point to any desired height. 



5. The tube is now to be hermetically 

 sealed, that is, closed by the fusion of 

 the glass at the upper extremity, which 

 for this purpose is previously drawn to 

 a capillary orifice. When it is intended 

 to free the tube entirely from air, which 

 is the best method with mercurial ther- 

 mometers, heat is again to be gently 

 applied to the bulb, which at the same 

 moment is to be softened by another 

 flame, and closed in the usual way, as 



soon as the mercury reaches the extre- 

 mity of the tube. When the ball has 

 cooled a little the sealing is rendered 

 more secure by fusing the glass more 

 fully around the top, so as completely 

 to obliterate the orifice. If the vacuum 

 be perfect, the mercury will fall to the 

 extremity of the tube on inverting the 

 thermometer, unless the calibre be ab- 

 solutely capillary ; in which case capil- 

 lary attraction will overcome the force 

 of gravity, and the mercury will retain 

 its position in the tube, in every situa- 

 tion of the instrument, 



Where there is a complete vacuum 

 in the tube, the mercury must be well 

 boiled before the sealing, as above di- 

 rected ; and when we choose a thermo- 

 meter, the ready falling of the mercury, 

 on inversion of the tube, is the best test 

 we can have that the mercury has been 

 well freed from air and moisture. This 

 vacuum is not, however, so essential to 

 the true action of the thermometer as 

 was once supposed. A thermometer 

 with a small dilatation of the tube 

 when sealed, containing some common 

 air, has lately been recommended as 

 preferable to the instrument with a va- 

 cuum on the surface of the mercury. 



M. Flaugergues* first called attention 

 to the fact, that when old thermometers 

 are placed in melting ice, they seldom 

 fall quite so low as the mark of freezing 

 on their stems, especially when the 

 whole air has been expelled from them. 

 This difference he found to amount 

 sometimes to 0.9 of a degree. The 

 same fact has been confirmed by MM. 

 De la Rive and F. Marcety}- and also by 

 Bellani $ and Arago. The writer of 

 this article possesses three thermome- 

 ters ; one very delicate, made by Rams- 

 den, and two well made instruments by 

 Lovi of Edinburgh, all which have been 

 in his possession upwards of a quarter 

 of a century. On lately placing them 

 in a vessel filled with pounded ice, in a 

 warm apartment, they all showed a 

 slight elevation of the freezing point. 

 That made by Kamsden has a capillary 

 tube and small spherical ball; the 

 other two have small pyriform bulbs, 

 and the mercury readily falls to the ex- 

 tremity of the tube on inverting them : 

 yet Ramsden's stood about 0.6 of a 

 degree above the freezing point, and the 

 other were just perceptibly above it. 



* Bibliothique Universelle, torn. xx. 1823. 



f Ib. torn. xxii. 



| Giornale di Fisica, torn. v. 



Aunales de Chimie, torn, xxxii. 



