320 DR. WALTER HosKMIAIN AND MR. SYDNEY L. ARCHBUTT ON THE 



Curve No. 100 i the cooling-curve of pure zinc. This merely shows the arrest 

 due to the freezing of the pure metal ; no evolutions of heat can be observed at lower 

 temperatures down to 200 C. 



No. 99 is the cooling-curve of an alloy containing 98 '8 per cent, of zinc; here a 

 small arrest is visible at 376 C., but nothing further. In No. 98 (97 '9 per cent, 

 zinc) three arrest points are to be observed : the highest is obviously the temperature 

 of initial freezing, the second is related to the lower of the two points in the previous 

 curve and occurs at the same temperature, while the third, at about 240 C., is a faint 

 indication of a heat evolution which becomes more marked in the following curves. 

 In the next, No. 95 (9576 per cent, zinc), we have two large points very nearly 

 merged together, one occurring at 383 C. and the other at 380 C. As the time 

 observations taken for these curves were made at intervals of 5 C., it would not 

 have been possible to separate these two points had the observer's attention been 

 confined to taking the data for the curve ; in practice, however, the exact temperature 

 at which the actual arrest of cooling occurs is carefully noted, and as an interval of 

 3 C. is represented by a distance of 24 mm. on the scale of the instrument, the two 

 arrests in the present alloy were readily distinguished. Curve No. 95 also shows a 

 decidedly more marked peak at 257 C. ; it will appear later that this is connected 

 with the points at 240 C. on No. 98, the lower temperature in the former case being 

 an example of the manner in which the temperature of a reaction is apt to be lowered 

 when that reaction is rendered faint by the extreme dilution of the constituent in 

 which it occurs. 



Curve No. 90 (8875 per cent, zinc) again shows three distinct peaks: the 

 temperature of the initial freezing has now risen to 426 C., the second point still 

 occurs at the constant temperature which lies between 377 C. and 381 C., while the 

 third point, now very large, is again seen at 257 C. 



Passing on to No. 84 (83'5 per cent, zinc) we find a curve with four points ; the 

 temperature of initial freezing has risen further to 450 C., and we again have arrests 

 at 381 C. and 257 C. In addition to these, however, there is now a very small 

 point occurring at a temperature of about 425 C. ; this again is the first sign of a 

 heat evolution which will be met with in a more vigorous form in succeeding alloys. 



In No. 76 (7 5 '29 per cent, zinc) we again have four points. The faint reaction 

 noticed in No. 84, at 425 C., has now developed into a marked heat evolution at 

 443" C., the temperature of the faint reaction in No. 84 being depressed by dilution. 

 We still have a small point in No. 76, at 380 C., and the peak at 257 C. is still well 

 marked. 



In No. 55 (557 per cent, zinc) the character of the cooling-curves has changed 

 considerably ; the heat evolutions at 443 C. and 257 C. can still be traced, but the 

 peaks found in previous curves at 380 C. are unrepresented here ; on the other hand, 

 a very small, but quite definite, peak is found at a temperature between that of 

 initial freezing and that of the reaction at 443 C., and appears to be the indication 



