In March of last year I communicated to the Society a paper giving a detailed account of the procedure adopted in an experimental investigation, which I had undertaken at the request of Lord Kelvin, on the heat of combination of pairs of solid metals in the formation of alloys, along with a statement of some results which had then been obtained.

The pairs of metals chosen were copper-zinc and copper-silver, and, briefly restated, the experiments were conducted as follows:—About half a gramme of alloy, reduced to powder by means of a file, was dissolved in dilute nitric acid, and an equal weight of a mixture of the same two metals in the same proportions was also dissolved under exactly similar conditions The difference between the initial and final temperature in each case is an indication of the heat of solution. If the two results are the same then there is no heat of combination in the formation of the particular alloy tested. But if the heat of solution of the mixture exceeds that of the alloy, then there must be heat of combination in the formation of the alloy equal to that difference. The experimental results, then, afford a means of approximately determining in absolute measure the heat of combination of the metals in the formation of an alloy.

The investigation has been continued since my last communication, and I expect to have it finished and to communicate the results to the Society either towards the close of the present session or at the beginning of the next.