54 



PHYSICAL SCIENCE 



through the colder surrounding fluid, carrying 

 heat with It ; by radiation, when heat passes 

 directly from one body to another, as from the 

 sun to the earth, without warming the Interven- 

 ing medium. Bearing In mind these three modes 

 of transference. Professor Dewar Invented a 



Fig. I. 



vessel in which a liquid gas can be kept, and the 

 effects of all three of these methods of heat- 

 transfer be reduced to a minimum — the now well- 

 known thermos flask. 



A double-walled glass bulb was taken, of one 

 of the forms shown in Fig. i, and the space be- 

 tween the walls exhausted of air to the completest 

 degree possible. This arrangement diminished 



