KNOBLAUCH ON RADIANT HEAT. 237 



" Or, the substance might also first be exposed to an Argand 

 lamp, and then to a heat of 21 2° F., and it be ascertained whether 

 rays of heat in both cases escaping to it permeate the diatherma- 

 nous media in the same or a different way. In the first case it 

 would be adiathermanous (compare Tables XV. and XVI. toge- 

 ther), in the second diathermanous (compare together Tables 

 XVII. and XVIII.)." 



Thus we have in reality obtained a new and certain means of 

 decidincj the question of the diathermancy of a substance, and are 

 in a condition for greatly increasing its delicacy, since there is 

 no further need of protecting the thermoscope from the heat of 

 the substance under investigation ; and the source of heat 

 may thus be allowed to act upon it to as great an extent as we 

 please. 



