THERMOMETRY AND CALORIMETRY 675 



fine bore in the stem. Like most other substances, mercury expands 

 when the temperature rises, and contracts when it sinks, and the amount 

 of expansion or contraction is shown by the rise or fall of the mercurial 

 column in the stem of the thermometer. The point at which the 

 meniscus stands when the bulb is immersed in melting ice or ice-cold 

 water is, on the centigrade scale, taken as zero ; the point at which it 

 stands when the thermometer is surrounded by the steam rising from 

 a vessel of boiling water is taken as 100 degrees. The intermediate 

 portion of the stem is divided into degrees and fractions of degrees. 

 When, now, we measure the temperature of any part of an animal with 

 such a thermometer, we place the bulb in contact with the part until 

 the mercury has ceased to rise or fall. We know then that the mercury 

 has ceased to expand or contract, and therefore that its temperature 

 is stationary, and presumably the same as that of the part. It is to 

 be noted that we have gamed no information whatever as to the amount 

 of heat in the body of the animal. We have only observed that the 

 mercury of the thermometer when its temperature is the same as that 

 of the given part expands to an extent marked by the division of the 

 scale at which the column is stationary. And we know that if the 

 mercury rises to the same point when the thermometer is applied to 

 another part, the temperature of the latter is the same as that of the 

 first part; if the mercury rises higher, the temperature is greater; if 

 not so high, it is less. The thermometer, then, only informs us whether 

 heat would flow from or into the part with which it is in contact if 

 the part were placed in thermal connection with any other body of 

 which the temperature is known. In other words, the temperature is 

 a measure of the heat ' tension,' so to speak; and difference of tempera- 

 ture between two bodies is analogous to difference of potential between 

 the poles of a voltaic cell (p. 724), or to difference of level between the 

 surface of a mill-pond and the race below the wheel. 



The temperature of an animal is measured in one of the natural 

 cavities, as the rectum, vagina, mouth, or external ear, or in the axilla, 

 or at any part of the skin. For the cavities a mercury thermometer 

 is nearly always used ; the ordinary little maximum thermometer is most 

 convenient for clinical purposes. The temperature of the skin may be 

 measured by an ordinary mercury thermometer, the outer portion of 

 the bulb of which is covered by some badly conducting material. An 

 uncovered thermometer, heated nearly to the temperature expected, 

 will also give results sufficiently accurate for most purposes, especially 

 if the bulb is flat or in the form of a flat spiral, which can be easily 

 applied to the surface. A theoretically better method, but more 

 laborious in practice, is the use of a thermo-electric junction, or a resist- 

 ance thermometer formed of a grating cut out of thin lead-paper or tin- 

 foil (Fig. 220). This is especially useful for comparing the temperature 

 of two portions of skin . The temperature of the solid tissues and liquids 

 of the body may also be measured or compared by the insertion of mer- 

 curial or resistance thermometers or thermo-electric junctions (p. 764). 



Calorimetry. The quantity of heat given off by an animal is generally 

 measured by the rise of temperature which it produces in a known 

 mass of some standard substance. Sometimes, however, as in the ice- 

 calorimeter of Lavoisier and Laplace, and the ether calorimeter of 

 Rosenthal, a physical change of state in the one case liquefaction of 

 ice, in the other evaporation of ether is taken as token and measure 

 of heat received by the measuring substance, the number of units of 

 heat corresponding to liquefaction of unit mass of ice or evaporation of 

 unit mass of ether being known. The unit generally adopted in the 

 measurement of heat is the quantity required to raise the temperature 



