Mr. J. Joly. 



(4.) The stirrup, which is of silver tarnished to blackness, and the 

 cross wires serving as the platform, without the catch water, gave : — 



0-1 0-1 0-2 01 0-2 

 Mean, 0*14 per 5 minutes. 



I have chosen these observations out of a large number of various 

 experiments as sufficient to give a good idea of how far this radiation 

 effect should be considered in observations made with this calorimeter. 

 But before dealing further with this question a few more experiments 

 must be added. 



(5.) Frequent observations with sensitive thermometers failed, even 

 under extreme conditions, to reveal radiation across the steam to the 

 walls of the calorimeter. These experiments were conducted in this 

 manner. A very delicate thermometer, removed from a hypsometer, 

 was coated thickly with lamp-black over the bulb, which measured 

 some 4 cm. in diameter. This was arranged so that the bulb 

 occupied the centre of a spherical brass calorimeter, single walled, and 

 left clean on the inside. Diameter, 14 cm. Steam was admitted and 

 the position of the mercury in the projecting stem of the thermometer 

 observed through a telescope. Cold water was now plentifully sprayed 

 over the surface of the calorimeter, but, although an observer atten- 

 tively watched the thermometer through the telescope while this was 

 being done, no change in the position of the thread of mercury could 

 be detected. All this time the abundance of evaporated water rising 

 from the outside of the calorimeter and the increased drip from the 

 inside showed that heat was rapidly passing through the walls. This 

 experiment is surprising, perhaps, but if the extreme smallness of the 

 effect indicated in the weight-experiments be considered, it need not, 

 I think, negative the suggestion that the increment is a radiation 

 effect. 



(6.) It remains to add what is perhaps the most conclusive experi- 

 ment on this radiation question — the experiment of coating the inner 

 walls of the calorimeter with lamp-black and comparing the rate of 

 increment with the rate obtaining in the absence of this coating. 

 The increase of surface with the lamp-black is the greater, as the many 

 tiny globules of water condensing from the steam and adhering to the 

 inner wall become each coated with the rough black deposit. Steam 

 was first passed through the space between the walls of the calori- 

 meter till all was heated. Had this not been done, the deposit of 

 lamp-black would have been washed away from the inner surface by 

 the copious condensation. When the steam had been some 5 or 6 

 minutes in the calorimeter, the current through the jacket was stopped 

 and observations begun. It is to be observed that the presence of 

 moisture between the walls would tend to diminish the protective 



