472 



Dr. G. R. A. Wright and Mr. C. Thompson. [Mar. 28 r 



position. To diminish oxidation, a reducing atmosphere was main- 

 tained in the tipper part of the lead-bath by covering it loosely with a 

 lid and passing a jet of coal-gas inside. In some few instances the 

 test-tubes were not weighted down with iron caps, so that the level of 

 the metal inside was above the top of the lead ; under these circum- 

 stances the upper part of the metal was largely heated by convection, 

 and in all such cases it was found that the lighter alloy retained some 

 of the heavier alloy interspersed through it, the convection currents 

 preventing complete separation by gravitation. Thus, for example, 

 two similar mixtures, containing about 23' 5 per cent, of tin, were 

 heated simultaneously for eight hours, one completely depressed so as 

 to avoid convection currents, the other raised so as to ensure their 

 production, with the following results : 



The figures obtained with the depressed tube are perfectly normal 

 (vide infra), whilst those obtained with the other are such as to show 

 that, owing to the convection currents set up, a little of the lighter 

 alloy was still intermixed with the heavier, whilst a considerable 

 amount of the heavier one was interspersed throughout the lighter 



one. 



Lead-bath Observations. 



As a check on the curves above described obtained with pipe-bowls, 

 we made another similar series of observations with mixtures contain- 

 ing equal quantities of lead and zinc and varying proportions of tin, 

 employing clay test-tubes heated in a lead-bath. The temperature of 

 the bath was ascertained from time to time by heating a bundle of 

 thick platinum rods in a clay test-tube in the bath and transferring it 

 to the calorimeter; the temperatures thus observed lay between 610 

 and 710, with an average of 646 for the whole series of ingots, 

 twenty-two of which were prepared, not one giving any markedly 

 abnormal results on analysis. The following figures were obtained, 

 several of the ingots being duplicates and the mean figures being 

 quoted. In all cases the time during which the mass remained molten 

 was about eight hours. 



