ANIMAL HEAT 573 



ordinarily contained in a thin glass bulb, the cavity of which is con- 

 tinued into a tube of very fine bore in the stem. Like most other 

 substances, mercury expands when the temperature rises, and con- 

 tracts 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 centi- 

 grade 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 gained 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 temperature between two bodies is analogous to differ- 

 ence of potential between the poles of a voltaic cell (p. 615), 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 tem- 

 perature of the skin may be measured by an ordinary mercury ther- 

 mometer, 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 resistance thermometer formed 

 of a grating cut out of thin lead-paper or tinfoil (Fig. 194). 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 mercurial or 

 resistance thermometers or thermo-electric junctions (p. 664). 



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 



