648 LECTURE Lir. 



semitransparent porcelain, containing a fusible metal, which may he com- 

 pared with the upper part of the mercurial scale, and then continued further; 

 and the expansion of such of the metals, as are difficult of fusion, affords an- 

 other mode of determining the highest degrees of heat. Mr. Wedgwood's 

 thermometer derives its properties from the contraction of a small brick of 

 prepared clay, which contracts the more, as the heat to which it is exposed is 

 higher : it may be extremely useful for identifying the degree of heat which 

 is required for a particular purpose : but for the comparison of temperatures 

 by an extension of the numerical scale, we have not sufficient evidence of its 

 accuracy ,to allow us to depend on. its indications ; and it is scarcely credible 

 that the operation of farnaces, of any kind, can produce a heat of so many 

 thousand degrees of a natural scale, as Mr, Wedgwood's experiments have led 

 liim to suppose; nor is the i^upposition consisteiat with the. observations of 

 other philosophers. ' )/i ■'[.': ■-.[,■•• [- .,;, 



Mercurial thermometers are in general hermetically sealed, the tube being 

 perfectly closed attheend, inordertoexcludedust, andtopreventthedissipation 

 of the mercury. When a standard therm ometer is to be adjusted, its freezing 

 point is readily fixed by immersing it wholly in melting snow or pounded ice; but 

 for the boiling point, some further precautions are required; the easiest method 

 appears to be, to immerse its bulb in an open vessel of boiling water, to cover it 

 with several folds of cloth, and to pour hot water continually over it ; for if it 

 were immersed to a considerable depth, the pressure would raise the temperature 

 of the boiling point, and if it were not covered, the mercury in the tube would 

 be too cold. Attention must also be paid to the state of the barometer; it must 

 either stand at 30 inches, or the place of the boiling point must be raised, when 

 the barometer is lower. than 30, and lowered when it is higher; the difference 

 of nine tenths of an inch either way requiring an alteration amounting to -,4^ 

 of the interval between freezing and boiling. This interval is subdivided, in 

 Fahrenheit's thermometer, into 180 degrees; in Reaumur's, into 80, and in 

 the centigrade thermometer of Celsius and of the French, into 100; and in 

 making the subdivision, care must be taken to examine the equality of the 

 bore throughout, by observing the length occupied by a detached portion cxf 

 mercury, and to allow for any irregularities which may have been thus de- 

 tected. The scales of Reaumur and of Celsius begin at the freezing point 

 of water; but in that of Fahrenheit the freezing point stands at 32"", the 



