1879.] Buff' s Experiments on the Diathermancy of Air. 19 



the needle of the galvanometer pointing to zero. When this was done 

 with the experimental tube exhausted, the plate of salt was pushed 

 up. A moderate deflection of the galvanometer was the consequence. 

 Repeated experiments made the heat intercepted by the salt 20 per 

 cent, of the whole radiation. This ivas the fraction stopped whether the 

 tube was exhausted or filled with air. This stoppage of 20 per cent, 

 exactly corresponds with the transmission of 80 per cent, resulting 

 from the previous experiments. 



I have also taken the trouble to repeat Professor Buff's experiments 

 with an apparatus similar to his own ; taking care, however, to avoid 

 the principal error into which I suppose him to have fallen. A glass 

 cylinder, A, fig. 3, 12 inches long and 2f inches in diameter, is mounted 

 on the plate p, p, of an air-pump. On it is placed a tin vessel C, with a 

 brass bottom, intended to contain the water which warms the bottom, 



or source of heat. A thermo-pile, P, is mounted on the air-pump plate 

 on which the cylinder stands, one of its faces being presented to 

 the bottom of the tin vessel. The conical reflector is abandoned, 

 a piece of tubing blackened within, and intended to cut off the 

 radiation from the sides of the vessel, being pushed over the pile. 

 Instead of bringing brass and glass into direct contact, as in the 

 apparatus of Professor Buff, a washer, w, w, of non-conducting india- 

 rubber, an inch and an eighth in thickness, separates the one from 

 the other. There is no chilling by cold water, and the distance of 



c 2 



