Resistance of certain Amalgams with Temperature. 447 



prevented a plate from being obtained which was thin enough 

 to photograph through. 



With cadmium amalgam only one point was shown at 

 which there is an evolution of heat. This corresponds to the 

 temperature at N, fig. 8. 



From the curve there shown it is seen that the liquid state 

 exists at a lower temperature when the substance is being 

 cooled than wheu its temperature is increasing. It is possible 

 than when solidification does take place, a compound is formed 

 which is unstable at ordinary temperatures, and when the 

 amalgam is allowed to stand for several days, the resistance 

 alters (see Section 5) through this unstable compound as- 

 suming a stable form. From the close similarity of the 

 heating and cooling portions of the curve (fig. 8) it seems 

 improbable, however, that we have a different compound 

 according as the amalgam is being heated or cooled ; the 

 most probable cause of the fall in resistance on standing is 

 that there is a slow alteration in the crystalline structure, 

 such as a breaking-up of crystals into more stable forms, a 

 change, in fact, similar to that which produces a displace- 

 ment of curves A and B in fio\ 1. 



With zinc amalgams no irregularity in the rate of cooling 

 was observed at points at all near the temperatures corre- 

 sponding to those at P, Q (fig. 3). With the exception 

 of those containing 4'8 per cent, and 9'5" per cent, of zinc, 

 the melting-point for each was over 100°. 



The expansion of the zinc amalgams was next tested. A 

 glass bulb carrying a narrow stem was nearly filled with the 

 amalgam, the remainder of the bulb and a part of the stem 

 being filled with oil or other non-volatile liquid having a 

 high boiling-point, this liquid acting as index. The whole 

 was freed from air by melting the amalgam under the liquid, 

 and then it was immersed in a bath and the position of the 

 index noted for different temperatures. Any irregularity in 

 the rate of expansion would thus be shown, especially as the 

 volume of the amalgam was considerably larger than that of 

 the liquid index. Plotting a curve giving relative volumes 

 and temperatures, it was found that the expansion was pro- 

 portional to the temperature up to 36°, after which the rate 

 of expansion gradually increased, and that when the tem- 

 perature was falling the volume was greater than when it 

 was rising. As in the case of the resistance, if the amalgam 

 was heated several times the initial and final volumes were 

 the same ; but if a cy^e was taken when it had been standing 

 for some weeks, the final w T as greater than the initial vo'ume. 

 Fig. 9 shows the curve obtained when it is taken to a tern- 



