284 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The authors of the present paper have used the same method, applying 

 it to measure the thermal conductivity of a certain grade of cast iron ; 

 but they have employed a thicker disk and thicker layers of copper. 

 The result of these changes has been to produce such uniformity of tem- 

 perature over each face of the disk, that calculation of the mean differ- 

 ence of temperature between the two faces of the disk has become an 

 exceedingly simple matter. 



The Iron Used, 



The disk was made from a slab of cast iron the origin and description 

 of which are well set forth in the following extract from a letter written 

 by Mr. A. C. Colby, the metallurgical engineer of the Bethlehem Iron 

 Company : — 



"Dear Sir, — In response to the instructions contained in your letter of 

 the 30th ultimo, I send to-day . . . the casting which has been made at 

 these works, and for which no charge will be made to you. I selected a high 

 silicon iron so as to make the casting free from any chill, and it is smooth 

 as can be obtained in a sand lined mould, and, I think, very close to the 

 dimensions you desire, namely, 12" X 4" X 1"- 



" In the following composition of the iron entering into this casting, the 

 sulphur and silicon determinations were made on a gate of the casting. 

 The other determinations are approximate, based on our daily analyses from 

 the furnace from which this casting was made : — 



Dimensions and Treatment of Disk. 



The diameter of the disk made from this casting was 10. OG cm.; its 

 mean thickness was called 1.787 cm.; the largest of nine measurements 

 at different places indicating 1.798 cm., and the smallest 1.776 cm. 



The copper plating of the disk was effected by giving it first a thin 

 coating from a cyanide of copper solution, such as is used by nickel- 

 platers in preparing iron to receive the nickel, and then finishing the 

 operation by use of a sulphate of copper solution. Much preliminary 

 experimenting in this process was done on a block of the cast iron be- 

 fore the disk was taken in hand, in order to make sure that a good 



