1893.] the Mechanical Equivalent of Heat, fyc. 9 



form of pressure regulator was constructed, and found to be fairly 

 successful, the variations in r during an experiment being generally 

 very small. 



The pressure in the space between the calorimeter and the walls 

 of the steel chamber was reduced, as a rule, to between 0*3 and 

 l'O mm.* 



If M is the capacity for heat of the calorimeter and its contents, 

 M/> will be the quantity of heat lost or gained per second by radiation, 

 &c, per unit difference of temperature, and provided the pressure is 

 unaltered, the value of M/> should be constant whatever the weight 

 of water. It was not until the close of our work that we were able 

 to obtain the value of M. We then found that the value of M/> 

 varied greatly with small changes of pressure, and our results 

 (although not necessary for the purposes of our investigation) are 

 interesting, since they bear out Bottomley's conclusion^ that there is 

 a sudden decrease in the loss by radiation when the pressure falls 

 below 0'5 mm. We extract from our paper the following table : — 



Table XXVI. 



Date. 



Experiments. 



Mass of 

 water. 



Pressure 

 in mm. 



Thermal 

 grams per 

 second. 





83—93 



139-78 



115 



0-0140 





41—50 



103 -01 



1-15 



0-0140 



„ 11 



51—54 



103 -01 



0-98 



0140 





94-102 



199 -67 



0-48 



'0138 



16—18 



103—110 



259 50 



0-48 



-0138 





79-80 



277 93 



0-44 



03 36 



„ 14—16 



58—59 



188 -07 



0-40 



-0134 



» 24, 25 



62—65 



188 -07 



0-37 



0032 





74—78 



277 '93 



37 



0-0132 



„ 26, 27 



66—71 



277 93 



0-37 



0-0131 



„ 17, 18 



60—61 



188 -07 



0-37 



-0131 





81—82 



140 -27 



36 



-0130 



The critical point of the curve deduced from the above table occurs 

 at a higher pressure, and the bend is somewhat sharper than is the 

 case with the curve given by Bottomley. 



If 



S0 1 



ct 



is the rate of rise due to the non-electrical supply, and 



to 

 then 



that due to the electrical supply, 



S0i 



Tt 



<!)■ 



* The pressures were ascertained by a McLeod's gauge, 

 f ' Phil. Trans.,' 1887, A. 



