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A MANUAL OF PHYSIOLOGY 



we have the means of finding out the temperature of any one body, 

 we can arrive at the temperature of any other by placing the two 

 in contact for a sufficiently long time, under the proviso that the 

 quantity of heat necessary to bring the temperature of the first body, 

 which may be called the * measuring ' body, to equality with that of 

 the second, is so small as not to make a sensible difference in the 

 latter. This is the principle on which thermometric measurements 

 depend. A mercurial thermometer consists of a quantity of mercury 

 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. 533), 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 tem- 

 perature expected, will also give results sufficiently accurate for most 



