THE THERMOMETER. 



145 



180, or, what is the same, 5 to 9, and to add to the result 32°, to allow for the 

 difference between the points at which the scale commences. To convert a 

 temperature on Fahrenheit into the corresponding temperature on the centi- 

 grade thermometer, it would be necessary to subtract 32°, and to diminish the 

 remainder in the proportion of 9 to 5. 



Thermometers are sometimes constructed for scientific purposes, to which 

 all the three scales are annexed. The reduction, however, of equivalent tem- 

 peratures, one to the other, is a measure of easy arithmetical calculation. 



Like all thermometers whose indications depend upon the dilatation or con- 

 traction of a liquid, the range of the mercurial thermometer is limited to the 

 points at which mercury freezes and boils. These points, however, as has 

 been already said, include between them a range of very great extent, through- 

 out, nearly the whole of which the indications of the thermometer are uniform. 

 The freezing point of mercury is placed at about — 39° of Fahrenheit, or 72° 

 below the freezing point. 



Mercury boils at 660°. Thus the range of the thermometer includes about 

 700° of Fahrenheit. The dilatations of the mercury, as it approaches its boil- 

 ing point, go on at a slowly-increasing rate ; but this increase is compensated 

 for by the expansion of the glass in which the mercury is contained, in such 

 a manner that the apparent dilatation shown by the actual ascent of the col- 

 umn in the tube is really uniform, and the same which w^uld take place if the 

 glass did not expand at all, and the dilatation of the mercury were absolutely 

 uniform. A thermometer intended to measure temperatures below the freez- 

 ing point of mercury may be constructed of spires of wine or alcohol. No 

 attainable degree of cold has ever yet reduced tiiis liquid to the solid state, and 

 a thermometer filled with it may be graduateJ, by comparison with a mercurial 

 thermometer, above the freezing point of mercury ; and its indications below 

 the freezing point will thus be rendered capable of comparison with the indi- 

 cations of a mercurial thermometer. 



Thermometers whose indications depend on the dilatation of air are rarely 

 used, except for peculiar purposes in which minute variations of temperature 

 only are required to be obtained. 



Since mercury boils at a higher temperature than any known liquid, it fol- 

 lows that no liquid thermometer can indicate higher temperatures than that of 

 660° Fahrenheit. To determine temperatures above this, the dilatation of 

 solids has generally been used ; and instruments founded upon this principle 

 are commonly called pyrometers. The changes of temperature are indicated 

 by the difference of the expansions of two metals. Such an instrument 

 would indicate all temperatures below that at which the more fusible metal 

 melts. 



In the use of the thermometer, and in the inferences drawn from its indica- 

 tions, care should be taken not to assume that the quantity of caloric introduced 

 into the bodies is represented by the degrees- of the thermometer. We shall 

 hereafter show that caloric may be introduced into a body without affecting the 

 thermometer at all, and also that different quantities of caloric introduced into 

 different bodies affect the thermometer equally. " Degrees of temperature" 

 are, therefore, to be carefully distinguished from the " quantity of heat ;" and 

 the thermometer must be understood as a measure of temperature, and not as a 

 measure of heat. When two bodies are said to undergo the same increase of 

 temperature, it is not meant that these two bodies receive the same increase of . 

 heat, but merely that they undergo such a change, with respect to heat, that '| 

 they are capable of causing a thermometer exposed to them to undergo the j 

 same degree of expansion. Again, if a thermometer be immersed in melting 

 ice, and observed to stand at the temperature of 32°, and the same thermome- 



VOL. II.— lO 



