ii THE THERMIC ECONOMY OF THE ORGANISM 65 



The thermometric scale most used is that known as the Centigrade or 

 Celsius scale, in which the two fixed points are : (a) the point corresponding 

 to the temperature at which ice melts ; (6) the point at which water boils 

 at a barometric pressure of 760 nun. of Hg. The former is the zero, the 

 latter the 100th division of the thermometric scale ; the interval between 

 the two is subdivided into 100 equal parts indicating the degrees of 

 temperature. 



The same two points are used in Reaumur's scale, with this difference, that 

 the higher division corresponds to 80 and not to 100, and therefore the whole 

 scale is divided into 80 degrees. In order to express the degrees of Reaumur's 

 thermometer in Centigrade degrees, it is only necessary to multiply the 

 Reaumur degrees by T25 (f ). 



The two fixed points of the scale of Fahrenheit's thermometer, which is 

 used in England, are the temperature of a freezing mixture of salt and 

 crushed ice, which corresponds to zero, and that of boiling water, which 

 corresponds to 212 degrees. Such a thermometer placed in melting ice 

 registers 32 ; hence the interval between the zero and the 100 of the Celsius 

 thermometer corresponds to 180 Fahrenheit ; in other words, 1 Fahrenheit 

 degree is equivalent to f of a Celsius degree. In order to express the results 

 obtained by a Fahrenheit thermometer in Centigrade degrees, it is necessary 

 to subtract 32 from the Fahrenheit degrees and multiply by f. 



For the measurement of still finer variations of temperature the thermo- 

 electric needles mentioned in Vol. III. Chap. I. are used. 



Another electric method of measuring very small differences of tempera- 

 ture is that known as the bolometrical method, which has been used success- 

 fully in researches upon exchange of energy. This method is based on the 

 principle that the resistance of a metallic circuit to the passage of an electric 

 current varies not only according to the nature and dimensions of the metal, 

 but also to its temperature ; the resistance increases in proportion to the 

 warming of the circuit 



The instrument to which Callender gave the name . electric resistance 

 thermometer consists of a thread of the purest platinum, 5 cms. long and 25 /* 

 thick, having a resistance of 8 ohm at C. This thread is wound on a very 

 thin plate measuring about 3 square mms., and isolated electrically by being 

 coated with wax; its ends are soldered with thin copper electrodes. The 

 total weight of the apparatus is about 4 mgrms. 



The apparatus is interposed in one arm of a Wheatstone bridge, of which 

 the other arm is joined to a second identical bolometer, the whole forming 

 part of an electric circuit, in which a galvanometer is placed. Only the first 

 bolometer is put in connection with the source of heat ; the second acts as a 

 compensator to eliminate the influence of the variations in the temperature 

 of the surroundings. Such a contrivance allows of the registration of varia- 

 tions of insVtrj and even of 3^77 f a degree. 



We will now consider the temperature of man, which is the 

 one of greatest interest to us. It is perhaps more variable than 

 that of the higher animals, changing, as it does, considerably, 

 according to the hour at which the observations are made. 

 Jurgensen (1873) found in a large number of observations that 

 the average temperature of the rectum in man was 37'34 by day 

 and 36'91 by night. The maximum and minimum temperatures 

 in a normal condition during rest were 37-7 and 36'2 respectively. 

 The corresponding figures found by Jager (1881) as the result of 

 observations on eleven normal individuals were somewhat lower 

 (3717 and 36*13), as were also those obtained by Richet and 



VOL. v F 



