54 SECTIONAL ADDRESSES. 



but it enables us to calculate the depa-ession of the freezing-points of a 

 large number of solvents. To do this it is necessarj^ to know the latent 

 heat of fusion of the pure solvent and the absolute temperature of the 

 freezing-point of the solution. That the numbers calculated are in very 

 close accord with the experimental values constitutes a strong argu- 

 ment in favour of the theory. From this time the study of alloys 

 began to make rapid progress. Laurie (Chem. Soc. Jour. 1888), by 

 measuring the potential difference of voltaic cells composed of plates 

 of alloy and the more negative element immersed in a solution of a salt 

 of one of the component metals, obtained evidence of the existence 

 of compounds such as CuZna.CuaSn. In 1889 F. H. Neville and I, 

 whilst repeating Raoult's experiments on the lowering of the freezing- 

 point of organic solvents, thought that it was possible that the well- 

 inown fact that alloys often freeze at a lower temperature than either 

 ■of their constituents might be explained in a similar way. In a pre- 

 liminary note communicated to the Chemical Society on March 21, 

 1889, on the same evening that Professor Eamsay read his paper on the 

 Ttiolecular weights of metals as determined by the depression of the 

 "v^apour pressure, we showed that the fall produced in the freezing- 

 point of tin by dissolving metals in it was for dilute solutions directly 

 proportional to the concentration. We also showed that the fall pro- 

 duced in the freezing-point of tin by the solution of one atcsiiic weight 

 of metal in 100 atomic weights of tin was a constant. 



G. Tannman about the same time (Zeit. PhysikaL Chemie, III., 44, 

 1889) arrived at a similar conclusion, using mercury as a solvent. 



These experiments helped to establish the similarity between the 

 liehaviour of metallic solutions or alloys and that of aqueous and other 

 solutions of organic compounds in organic solvents. That our experi- 

 ments were correct seemed probable from the agreement between the 

 observed depression of the freezing-point and the value calculated from 

 Van't Hoff's formula for the case of those few metals whose latent 

 heats of fusion had been determined with any approach to accuracy. 



Our experiments, subsequently extended to other solvents, led to 

 the conclusion that in the case of most metals dissolved in tin the 

 molecular weight is identical with the atomic weight ; in other words, 

 that the metals in solution are monatomic. This conclusion, however, 

 involves certain assumptions. Prof. Eamsay 's experiments on the 

 lowering of the vapour pressure of certain amalgams point to a similar 

 conclusion. 



So far our work had been carried out with mercury- thermometere, 

 standardised against a platinum resista.nce pjTometer, but it was evident 

 that, if it was to be continued, we must have some method of extend- 

 ing our experiments to alloys which freeze at high temperatures. The 

 thermo couple was not at this stage a reUable instrument ; fortunately, 

 however, Callendar and Griffiths had brought to gi-eat perfection the 

 electrical resistance pyrometer (Phil. Trans. A, 1887 and 1891). Dr. 

 E. H. Griffiths kindly came to our aid, and with his help we installed 

 a complete electrical resistance set. As at this time the freezing-points 

 of pure substances above 300° were not known with any degi-ee of 

 accuracy, we began by making these measurements : — 



