OF IRON", COPPER, AND GERMAN SILYER. 



543 



Iron. 



Distance from origin 

 in inches.* 



Temperature Excess °C. 



11/6/86. 

 Bar not cooled. 



13/4/87. 

 Bar cooled 46 in. from origin. 







3 



6 



9 



15 



21 



27 



33 



45 



229-3 



1625 



11845 



87-6 



5035 



29-8 



18-25 



11-4 



46 



229-35 



1633 



11985 

 89-5 

 510 

 29-95 

 17-65 

 9-6 



-l-05f 



* The hole in the har nearest the heated end is taken as origin. 



t The — sign indicates that the actual temperature of the har at this point was below that of the 

 short har standing in the vicinity. 



Cooling Experiment. — The chief difficulty hitherto in this part of the 

 experimental method has been the oxidation of the short bar when heated to a 

 high temperature, This introduces an uncertainty into the final results owing 

 to the necessity of applying a value of the " rate of cooling " of the short bar 

 at a particular temperature, to a portion of the long bar whose surface at 

 that temperature is generally in a different state from that of the short bar 

 at the same temperature. And although a correction may be applied to the 

 ordinates in the curve of cooling in order to remove this, its application 

 must in many cases be a matter of doubt. 



In this work, however, this uncertainty has been almost entirely removed 

 by the nickel-plating of the bars ; the difficulty arising from an oxidised surface 

 never even presented itself, for at the end of each experiment the surface of 

 the short bar on cooling had retained its original brightness, and had, except 

 in a slight manner to be yet noted, in no way been affected. So that the 

 condition which is necessarily involved in the deduction of the conductivity, 

 viz., that the surface of the long bar at any particular temperature should be 

 the same as, or, at least, strictly comparable with, that of the short bar when 

 at the same temperature, was very fully realised. 



Each of the short bars experimented upon was so placed, on bearings, in a 

 rack, that it was possible to rotate it, on its long axis, while it was being heated. 

 The temperature of the bar was raised by placing beneath it a row of 50 very 

 small Bunsen burners. The heating was proceeded with as cautiously as 

 possible, and generally (where it finally reached about 270°) occupied about 2 

 hours. This, so far, guaranteed an equable distribution of temperature through- 



VOL. XXXIII. PART II. 4 L 



