318 DR WALTKR ROSENHAIN AND MR. SYDNEY L. ARCHBUTT OX THE 



were hot the temperature fell at a rate of 1 C. per second, the rate of cooling 

 decreasing as the alloys cooled, until it became 1 C. in 12 seconds. It was found 

 that this rate of cooling was too rapid and for the data used in the present paper a 

 series of curves were obtained at much slower rates of cooling, ranging from a rate of 

 fall >f temperature of 1 C. in 4 seconds when the alloys were hot (near 650 C.) to a 

 rate of 1 C. in 20 seconds when the alloys were cool (near 200 C.). 



Tli is slow rate of cooling was adopted because it was found that certain of the 

 reactions or inversions which occur in these alloys are to a considerable extent 

 suppressed by the more rapid rates of cooling ; even the rate adopted does not allow 

 tln-M- reactions to be completed, but they occur to a sufficient extent to allow 

 unmistakable signs of their existence to appear on the cooling-curves. For the 

 purpose of studying the alloys under conditions of complete equilibrium, it was, 

 therefore, necessary to adopt the method of microscopic examination of samples which 

 had been subjected to prolonged heating. 



As already indicated, the temperature measurements were made by means of a 

 thermo-couple and potentiometer. One thermo-couple, composed of wires of platinum 

 and platinum with 10 per cent, of iridium, has been used throughout the research ; 

 this junction has never been broken or repaired, and it has been carefully protected 

 from injury by such causes as unnecessarily high temperatures or contact with 

 furnace gases or other injurious substances. At the beginning of the research, and 

 frequently during its progress, this thermo-couple has been calibrated by using it to 

 determine the freezing-points of a series of standard pure metals. In these calibra- 

 tions the thermo-couple was used in the same small furnace, and with the same fire- 

 clay protectors, as were employed in taking the cooling-curves, and similar rates of 

 cooling were used. The temperature-E.M.F. curves obtained from these calibrations 

 could therefore l>e applied direct to the readings of the cooling-curves without the 

 introduction of any corrections. At the conclusion of the research the thermo-couple 

 gave readings which agreed with the original calibration within 1 C. 



The use of very slow cooling, and the fact that the alloys were allowed to solidity 

 in a perfectly undisturbed manner, would seem to involve the risk that the 

 observations might be vitiated by errors due to surfusiou. Surfusion phenomena can, 

 however, be detected readily with the apparatus used in this work, by the occurrence 

 of a rise of temperature following the first arrest of cooling ; although small rises of 

 temperature of this nature were frequently observed, the large mass of alloy used, 

 mid tli slow rate of cooling, always resulted in a prolonged arrest at a definite 

 maximum temperature, and the manner in which the observed points fall upon 

 smooth curves or upon straight lines shows that errors from surfusiou have been 

 avoided. 



In the study of the cooling-curves, the cooling of the same ingot has in many cases 

 IH-.-M repeated three aud even four times in order to eliminate the possibility of 

 mistaking small irregularities due to experimental error for minor heat evolutions' 



