240 



Sir James Dewar 



[Jan. 16, 



and polished rock-salt were successfully employed, both these sub- 

 stances being very transparent to dark heat rays. 



These results were demonstrated as follows : — 



(1) Heat transference through a thin membrane. In the parallel 

 beam from an arc lamp was placed an unsilvered glass spherical 

 vacuum vessel, 5 to 6 inches diameter, filled with filtered liquid air. 

 In the focus of the rays thus obtained pieces of black paper were 

 readily ignited. The same result followed when a thin membrane of 

 stretched india-rubber was interposed, as also with a plate of polished 



Fig 



rock-salt. With an india-rubber membrane less stretched, and there- 

 fore thicker and not so clear, there was difficulty in getting the 

 paper to glow, while a dull and still thicker membrane absorbed the 

 bulk of the heat, and no appreciable effect could be obtained. 



When a pad of wool soaked in liquid air was drawn across a 

 stretched rubber membrane, puckering was produced by the expan- 

 sion of the india-rubber. The opposite effect was demonstrated by 

 means of an india-rubber balloon distended by water, and supported 

 in a large glass funnel. By carefully pricking the balloon with a 



