250 



Prof. S. U. Pickering on a New Form 



opened ; but I have found it convenient, and often necessary, 

 not to wait till absolute identity is attained, but to open the 

 doors when there may be a difference sometimes of as much 

 as o, 003 between the two temperatures. This difference, when 

 unavoidable, was entirely due to local circumstances which 

 are independent of the apparatus itself, chiefly to its position 

 with regard to the windows in the room, one division of it 

 being nearer the window than the other. The correction ne- 

 cessary on account of this difference (often nil) is so insigni- 

 ficant that it does not require any knowledge of the heat- 

 capacities of the liquids, and necessitates an accurate comparison 

 of the readings of the two thermometers throughout but a very 

 small portion of their scales (only two or three millimetres), 

 since the initial temperatures in the individual experiments of 

 any series need never differ by more than this amount. 



The following details of an experiment will render the 

 method of procedure clear. 





Left-hand Division. 



Right-hand Division. 



Water. 



Thermometer '61. 



CaCl 2 sol. Thermometer '08. 



Time. 



Eeading. 



Eate of 

 cooling. 



Time. 



Eeading. 



Eate of 

 cooling. 



6 



14565 mm.* 





30 



151-33 mm.* 









•48 



1 





•28 



1 



145-17 „ 



•27] 



1 30 



151-05 „ 



•25) 



2 



14490 „ 



i-27 

 •27 J 



2 30 



1 



150-80 „ 



i=-23 

 •20 J 



3 



144-63 „ 





8 30 



150-60 „ 





5 30 



19895 „ 



•60 s ! 



6 



195-50 „ 



•50^) 



6 30 



198-35 „ 



! 



7 



195-60 „ 







•70}- = 62 







•50 }>=-48 



7 30 



19765 „ 



•55 J 



8 



194-50 „ 













•45; 



8 30 



19710 „ 





9 



19404 „ 





* 1 millimetre represents about 0°01 C. 



The rate of cooling here became constant after V 0" '. The 

 last reading of the thermometer for the initial temperatures 

 was taken at 3' 30'', and the doors were opened by 4/ 0" Up 

 to this time the first-determined rates of cooling would hold 

 good, and the reading of thermometer '61 at this moment 

 would be [1 44*9 (the mean of the three previous readings) 

 ,= (■27x2) = ] 141-36 millim.; while that of '08 would be 



