754 PLUCKER ON THE DETERMINATION OF THE 



externally with the wet finger. The current was not interrupted 

 during these determinations of weight. The weights are those 

 directly required for the separation ; correction appears unne- 

 cessary, because the cup with the sand only is scarcely at all 

 affected by the electro-magnet : — 



Weights for separation. 



grnis. 



153-70 



158-25 



159-80 



161-70 



162-40 



163-16 



166-75 



We thus see how the magnetism of sheet iron, on cooling to 

 the temperature of the room, continually increases. The dif- 

 ference amounts to 8 per cent, of the intensity determined at this 

 temperature. 



61. Peroxide of iron was next examined. For this purpose, 

 instead of the brass cup, I took a somewhat smaller one of 

 porcelain, which was suspended to the balance in a brass ring. 

 The peroxide, about 25 grms., was heated to 752° F. at least; 

 but its temperature at the first weighing had sunk to about 

 572° F. Nine weighings, immediately following each other, were 

 taken ; and after the ninth weighing the temperature of the per- 

 oxide was 84° F. The current was excited by three of Grove's 

 cells. The attraction of the empty porcelain cup, which amounted 

 to 0-14 grm. only, has been deducted in each case: — 



grms. 



1. Weighing 52-01 



2. ... 56-97 



3. ... 59-54 



4. ... 63-04 



5. ... 65-96 



6. ... 67-61 



7. - 67-91 



8. ... 68-67 



9. ... 69-61 



The magnetism of the peroxide of iron, at the highest tempera- 

 ture observed, is thus rather more than 25 per cent, less than at 

 the temperature of 74° F. For the same temperatures it dimi- 



