

EUTECTIC RESEARCH: THE ALLOYS OF LEAD AND TIN. 91 



It will be seen that the largest difference between calculated and analytically 

 determined composition never exceeds 0'3 per cent., and this degree of accuracy is 

 adequate for the purposes of these experiments as a whole. Those alloys whose 

 composition is required to be more accurately known have been analysed. 



Experimental Methods. The " thermal analysis " of the alloys was carried out by 

 means of cooling-curves taken both by the "inverse-rate" and the differential methods. 

 Thermocouples of platinum and platinum-iridium were used for the most part, 

 although for some special purposes, notably the cooling- and heating-curves at very 

 low temperatures, a Constantan-iron couple was used. The indications of the thermo- 

 couples were observed by means of the delicate potentiometer installed at the 

 Laboratory. The time observations in the case of the inverse-rate curves were made 

 with the aid of a chronograph indicating seconds by means of an electrical connexion 

 with the standard clock of the Laboratory. The differential curves were obtained by 

 observations of the deflection of the differential galvanometer on the same scale us 

 that used for the galvanometer connected with the potentiometer already referred to. 

 The details of this apparatus and of the method of using it have been published 

 previously,* so that no further reference is required here. The thermocouples were all 

 carefully calibrated at frequent intervals during the progress of the research, both by 

 comparison with the standard couples of the Laboratory and by direct determination 

 of well-known fixed points, viz., the boiling-point of water and the freezing-points of 

 pure tin and pure lead, the former being taken as 232 C. and the latter as 328 C. 

 Graphic interpolation on large-scale curves was used to obtain the values of 

 intermediate points on the thermocouple scale. These repeated calibrations gave 

 remarkably constant results, and the indications of the platinum couples may therefore 

 be regarded as accurate to within 0'8 of a degree, although it must be admitted that 

 a slightly greater error may be introduced by the small difference of temperature 

 which is liable to exist between a thermocouple and the mass of metal in which it is 

 immersed. 



The microscopic examination of alloys containing more than 30 per cent, of tin 

 presented no serious difficulty, as it was found possible to polish these alloys by the 

 ordinary methods used for the preparation of soft metals. Alloys richer in lead, 

 however, proved intractable until special means were employed for polishing them. 

 These special means, adopted on the advice of M. F. OSMOND, consisted in the 

 preparation of laevigated oxide of chromium in the manner described by 

 LE CHATELIER.! When the finer grades of this material are used sparingly on 

 " beaver-cloth " discs driven at moderate speeds, it is found possible to prepare 

 polished surfaces of the alloys close to the lead end of the series for microscopic 



* CARPENTER and KEELING, "The Range of Solidification and Critical Ranges of Iron-Carbon 

 Alloys," 'Journal of the Iron and Steel Institute,' vol. i. ; also STANSFIELD, 'Phil. Mag.,' vol. xlvi., 

 pp. 59-82. 



t LE CHATELIER, ' Metallographist,' vol. 4, January, 1901. 



N 2 



