440 WRIGHT: A NEW DIP CHART 



As closely as can be measured, the transformations Acs and 

 Avs begin at the same temperature, 894°C. (see line^i^ of fig. 2); 

 and as given by the resistance measurements, Acs and Avs each 

 extend over the considerable temperature interval of 25°C. 



These resistance measurements therefore show that A2 is a, 

 strictly reversible transformation and that ^3 is a transformation 

 taking place at a higher temperature on heating than on cooling. 

 Evidently, the two types of transformation are fundamentally 

 different. 



The experiments here described are in agreement with the 

 thermal observations previously recorded (see Reprint 213), 

 although the position of maximum absorption or evolution of 

 heat does not appear to coincide exactly with the temperatures 

 at which the electrical resistance is changing most rapidly either 

 Sit A 2 or A3. The type of phenomenon is however the same as 

 given by either method for A2 and A3 respectively. 



Whether or not either or both of these critical ranges, A 2 

 and A3, is to be considered an "allotropic point" will depend on 

 the definition of allotropy, about which there does not yet appear 

 to be agreement. The reversible thermal and electrical behavior 

 at A2 appears to be somewhat similar to that of a pure substance 

 at its melting point, while at A3 there is a progressive change 

 with temperature of the electrical and thermal properties which 

 are not reversible, the reaction taking place at a higher tempera- 

 ture on heating than on cooling. The A3 change is certainly 

 associated with recrystallization while no crystallographic change 

 has as yet been found at A2 which is also the temperature associa- 

 ted with the abrupt, reversible change of iron from the ferro- 

 magnetic to the para-magnetic states. 



GEOLOGY. — A new dip chart. Fred Eugene Wright, Geo- 

 physical Laboratory. 

 In both structural and mining geology the problem frequently 

 arises to determine the direction, on a given vertical section, of 

 the trace of a bed or plane of known dip and strike. This direc- 

 tion is the apparent dip of the stratum or vein as seen in the 

 vertical section. This problem can be solved either by a purely 



