344 



MODES OF DIFFUSING HEAT. 



FIG. 258. 



burned without rendering the other end uncomfortably 

 warm. We thus see that some ^substances are good con- 

 ductors of heat while some are not. Thrust a silver and 

 a German silver spoon into the same vessel of hot water, 

 and the handle of the former will become much hotter 

 than that of the latter. 



(a.) Fig. 258 represents a bar of iron and one of copper placed 

 end to end so as to be heated equally by the name of the lamp. 

 Small balls (or nails) are fastened by wax to the under surfaces of 

 the bars at equal distances apart. More balls can be melted from 

 the copper than from the iron. The number of balls melted off, not 

 the rapidity with which they fall, is the test of conductivity. The 

 rapidity would depend more upon specific heat. 



(&.) Relative thermal conductivity of some medals : 



Silver 100 



Copper 74 



Gold 53 



Brass 24 



Tin.. 15 



Iron 12 



Lead 9 



Platinum 8 



German silver 6 



Bismuth . 2 



The above-named metals arrange themselves in the same order 

 with reference to the conduction of electricity, silver being the best 

 and bismuth the poorest. This relation suggests a similarity of 

 nature between these two "agents. 



54O. Conductivity of Fluids. Liquids and 

 aeriform bodies are poor conductors of heat. The 

 surface of a liquid may be intensely heated without sensibly 

 affecting the temperature an inch below. 



