CONSTITUTION AND TEMPERATURE ON MAGNETIC SUSCEPTIBILITY. 119 
substance, at room temperature. If the substance were a liquid it was cooled by 
liquid air, and after about 1 hour observations were begun and taken at a large 
number of temperatures as the substance gradually warmed to room temperature. 
A series of such observations occupied a period of 10 hours. Frequently the heating 
was too slow to allow the complete interval to be bridged during the day, and in such 
cases the curve was completed the following day. It will be observed that the lag in 
temperature of the substance behind the thermocouple, with such a small rate of 
cooling, will be small, and this lag will be uniform so long as there is no sudden 
change of the state of the substance. At temperatures considerably higher or lower 
than the temperature of fusion, the lag will generally be greater than it is at the 
fusion-point itself. It can easily be seen from the curves given below that the effect 
of this will be to lessen the difference of susceptibility between the liquid and the 
crystalline states. The actual difference of susceptibility cannot, therefore, be 
attributed to this source even if the lag were considerable. A similar series of 
readings was taken for the phial alone, which, corrected for the variation of the 
susceptibility of the contained air with change of temperature and subtracted from 
the readings obtained above, will give the variation of the specific susceptibility of 
the. substance with temperature. 
Similar experiments have been made, using hot water and other liquids in the 
Dewar tube, but it was difficult to take readings at temperatures higher than 100° C. 
