326 DR WALTER ROSENHAIN AND MR. SYDNEY L. ARCHBUTT ON THE 



sheaths, by separating the two reacting bodies, considerably retard the further 

 progress of the reaction, which can then only take place by diffusion through the 

 sheath. An indication of the composition of this compound has already been obtained 

 by the determination of the end of the eutectic line, which has been found to lie at a 

 concentration of about 78 per cent, of zinc, and this corresponds approximately with 

 the composition of the compound AlZn 3 which contains 7 8 '3 5 per cent, of zinc. It 

 was not found possible to apply the method of approximate quantitative estimation 

 of the heat evolutions along the line CH for the purpose of determining the concen- 

 tration at which this heat evolution attains its maximum value. In the first place, 

 the peaks representing these heat evolutions on the cooling-curves lie so near the 

 large peaks representing the initial freezing of the alloys that the precise shapes of 

 these smaller peaks cannot be ascertained even with that degree of approximation 

 which was reached in the other case ; further, this reaction is not complete when the 

 alloys are cooled even at the slowest rate practicable for cooling-curve observations, 

 and no reliance can be placed on the quantities of heat evolved by a series of heat 

 evolutions which might proceed to a greater or lesser degree of completion in the 

 various alloys of the series. 



In the absence of the kind of evidence just referred to, it is fortunate that it is 

 possible to show an intimate connection between the heat evolutions which take place 

 along the line CH and those which occur at a temperature of 256 C. along the 

 line IL. This connection was established in the first place by means of cooling-curves 

 of a number of the alloys taken first by allowing them to cool at the standard slow 

 rate from fusion, and again after prolonged heating at 430 C. It was found that 

 the size of the peak at 250 C. increased very much after the alloy had been exposed 

 to prolonged heat at a temperature just below that of the line CH. This is illustrated 

 in fig. 7, where cooling-curves of alloys Nos. 70 and 75 are given. The peaks on the 

 cooling-curves of these alloys when cooled slowly from fusion are shown on the curves 

 marked (a) and (<), while the corresponding peaks on the same alloys after prolonged 

 annealing at 430" C. are shown on the curves marked (b) and (d). The curves them- 

 selves show that the rate of cooling was practically identical in all four cases. This 

 observation indicates that the magnitude of the lower heat evolution is directly 

 dependent upon the extent to which the reaction along the line CH has been allowed 

 to take place, and the inference is justified that the heat evolutions along the line IL 

 are due to a reaction occurring in the compound which is formed along the line CH. 

 rhis inference will be more completely established by the microscopic evidence to be 

 described below. 



The connection between the heat evolutions along the line IL and those along the 

 line CH having been established, an attempt was made to utilize the measurement of 

 the quantities of heat evolved by the reactions at 256 C. for the purpose of locating 

 the maximum of that reaction, a maximum which would coincide with the maximum 

 of the heat evolutions along the line CH. An attempt to do this was first made by 



