OF ARTS AND SCIENCES. 47 



A bar of stub steel, of the same dimensions, was found to have a 

 magnetic moment of 8, 750, 000. This variety of steel is used for the 

 common grade of tools, and is comparatively soft. The magnetic 

 moment of a bar of ordinary cast nickel was then obtained. Both 

 bars were highly tempered to render the conditions of comparison as 

 nearly equal as possible. 



With the same formula the magnetic moment of a bar of cast 

 nickel was found to be only 30,330; that is, about one two-hundred- 

 and-t\Ventieth as much as the moment of Fleitman's nickel. 



These numbers are, of course, not perfectly exact, for the changes 

 of magnetism in bars from time to time preclude perfect exactness ; 

 still the relation of the magnetic moments may be considered very 

 accurate. 



This result is certainly surprising; that the addition of ^ per cent 

 of magnesium to a bar of nickel should increase the masfnetism 220 

 times shows that change of structure in a metal increases its magnetic 

 capacity. 



No. XXIV.— THERMAL CONDUCTIVITY OF GLASS AND 



SAND. 



By C. B. Penrose. 



Presented June 9, 1880. 



In determining the conductivity of glass I used the same method 

 that Forbes employed in determining the conductivity of iron. 



A bar of glass is maintained, at one end, at a constant temperature. 

 When the bar has reached a permanent state of heat, that is, when 

 the amount of heat received by any portion exactly equals the amount 

 given out by that portion, the temperature of a number of points on 

 the bar are determined. The bar is taken so long that the heat at the 

 heated end will not be sufficient to raise the temperature of the other 

 end above that of the air. The temperatures determined are laid off 

 as ordinates, the abscissas. being the corresponding lengths of the bar. 

 The equation of the curve thus formed can be determined. Then 

 an exactly similar bar to the preceding is heated to a known tempera- 

 ture, and as it cools the temperatures are taken every minute, and 

 thus the loss of temperature per minute is determined. 



