366 



Mr. J. Joly. 



suspended water in the steam is negatived by this constancy of 

 weight. Probably such dust is all deposited in the steam pipe 

 shortly after leaving the boiler. 



Mist or water-dust doubtless is present at first, due to the condensa- 

 tion produced in heating up air in the calorimeter. Some of this is 

 thrown upon the substance, but this will be small in quantity, 

 probably inappreciable, and acts against what error is due to radia- 

 tion from the incoming steam to the substance. This last is an error 

 analogous to the error of transference in other methods, but with 

 easily attainable rapidity of air displacement probably far less. 



The degree of accuracy attainable in effecting the measurements 

 remains to be considered. Of these, it is evident that in obtaining 

 the temperature limits great accuracy is attained by the use of 

 thermometers of no extraordinary sensitiveness. If and t 2 ° are 

 reliable to part of a degree, over a range of 80° we are observing 

 to g-^Q- part the quantity to be measured. 



As regards the observation of w, the weight of water condensed, 

 obviously the degree of accuracy attainable depends on the sensitive- 

 ness of the balance and the magnitude of w. It is also necessary, 

 however, to consider in some detail how far it has been found possible 

 to perfect the mechanical conditions attending the observation of the 

 weight of the substance while immersed in steam. 



It is evident in the first place that as regards the danger of intro- 

 ducing error into the equilibrium of the balance by heat radiating to 

 its parts, the difficulty may at once be got over by arranging things 

 so that the calorimeter is outside the balance case, as beneath it, the 

 balance standing on a shelf, suppose, and a wire taken down through 

 apertures in balance floor and shelf. In the form already described, 

 indeed, I have detected no error due to unequal heating of the beam, 

 although I have frequently looked for it by unhooking the substance 

 during experiment and testing for want of equilibrium. This is due 

 to the protection afforded by the non-conducting material of the tunnel. 

 It was necessary, however, to retain but one pan, as heat conducted by 

 the rails through the floor of the calorimeter set up a draught tending 

 to draw up the pan. 



I have detected no error, measurable by the balance, due to con- 

 densation on the wire for suspending the substance, where it issues 

 from the calorimeter. If this wire be closely observed it will be seen 

 that a little bead of water forms on it just above the mouth of the 

 aperture, and although occasionally reinforced by another little bead 

 slipping down to it, it soon reverts to its original bulk, probably by 

 evaporation promoted by heat conducted along the wire. With the 

 size of wire used — about 0*15 mm. in diameter — I do not think this 

 bead of water ever exceeds half a milligram in weight, and, as I say, 

 the balance shows no sign of any gradual increment of weight. This 



