On the Steam Calorimeter. 



227 



every case were consistent from one apparatus to another. In large 

 or small calorimeters, with varying dimensions of steam-pipe, boiler, 

 or exit tube, the consistency of result was maintained. Thus the 

 single calorimeter described further on will be found very different 

 in the disposition of its parts from the form designed for use within 

 the case of a balance as described in my former paper. In the course 

 of some recent experiments on the mineral sulphides, I desired to 

 check in some cases former results, by dealing over again with the 

 specimens dealt with in the first calorimeter. I have collected these 

 cases here. 



Galenite, cleavable fragments — 



f Former result 



I Another former experiment . . . 



L Experiment in new calorimeter 



Another specimen, close-grained — 



j" Former result 



\ Result in new calorimeter .... 



Sphalerite, cleavable, high 

 lustre — 



f Former result 



1 Result in new calorimeter 



Pyrite, two interpenetrated 

 cubes — 



f Old result 



\ Recent result 



W. 



56 -967 

 56-951 

 56 -951 

 56 759 



87 -840 

 93 -817 



45 -765 

 45-755 



46 -710 

 46-710 



10 *50 



10- 98 



11- 77 

 18 05 



13-27 

 17-65 



11-80 

 16-25 



10-50 

 17-50 



99 -90 

 99-76 

 99 -70 

 100-00 



99-60 

 100-10 



100-18 

 99-80 



99-90 

 100 -00 



0-496 

 0-493 

 0-487 

 '4545 



0-6955 

 -7141 



0-863 

 0-8151 



1-016 

 -9375 



As regards what is probably occurring in the calorimeter, it is con- 

 venient to consider the duration of an experiment as divided into 

 a period in which the substance is rising in temperature, and a period 

 in which, having attained the temperature of the steam, it hangs 

 surrounded by an atmosphere of saturated vapour, while its weight i& 

 being determined. I briefly epitomise, in part from my former 

 paper, the following considerations. Throughout the first period : — 

 (a.) The film of water, which is almost immediately formed upon the 

 substance, is having its outer surface continually renewed by precipi- 

 tation of fresh steam, and hence presents a surface but slightly 

 cooler than the surrounding steam. (&.) This film of water will be 

 very adiathermanous to radiation from water vapour, so that most of 

 the steam will condense, not by radiation, but by contact with the 

 slightly cooler outer service of the water-layer. (c.) A reduced 

 pressure will obtain, or tend to obtain, around the substance, giving 



