826 



Professor Dewar 



[April 1, 



the use of finely divided solids, ever so effective as a high vacuum, 

 in an empty tube, the glass surfaces being silvered. This is seen by 

 reference to results recorded in Tables Nos. 1, 2 and 3, where the 

 insulation is increased in the proportion of more than 1 to 7, which 

 is decidedly better than anything reached by the use of powders. 



It will be noted that the use of silica and charcoal to fill up the 

 annular spaces between the walls of these silver-coated vacuum 

 vessels has produced very different results from those recorded in 

 the former experiments with plane glass surfaces. Instead of the 

 heat insulation being increased from 4 to 5 times by the use of such 

 powders, it is now only very slightly benefited. This suggests that 

 the finely divided solid affects chiefly the combined radiation auJ 

 conduction factors. 



A further set of experiments was made with similar vacuum tubes, 

 replacing the powders by metallic and other septa (Fig. 4). Various 

 papers coated with metallic powders in imitation of gold and silver 

 which are in common use, were compared with black paper and a com- 

 parison made between the use of sheet lead and aluminium, all under 

 similar conditions. 



The following tables express the comparative results of the differ- 

 ent experiments. 



(1) 

 Vacuum space empty, not sil- 

 vered 1 



8ame space unexhausted .. .. 0*25 

 Vacuum sf^ace empty, silvered 

 on both surfaces 7 ' 4 



(2) 



Vacuum space empty, silvered on 



inside surfaces 1 



Silica in silvered vacuum space 1 • 1 



Empty silvered vacuum 1 



Charcoalin silvered vacuum 1*25 



Vacuum space unsilvered 1 



„ silvered inside 5 



„ in annular space with glass test-tube silvered .. G 



(4) 



Vacuum space empty 1 



Tliree turns silver paper, bright 



surface inside 4 



Three turns silver paper, bright 



surtace outside 4 



(6) 

 Vacuum space empty 

 Three turns gold paper, gold out- 



side 



Some pieces of gold leaf, put in 

 so as to make contact between 

 walls of vacuum tube 



1 



4 



0-3 



(5) 



Vacuum space empty 1 



Three turns b^ack paper, black 



outside 3 



Three turns black paper, black 



inside 3 



(7) 



Vacuum space empty 1 



Three turns, not touching, of sheet 



lead 4 



Tluee turns n(*t touching, of sheet 



aluminium 4 



The experiments show that liquid air can be conveniently used 

 to study many important problems of heat transmission. 



