SCHEELE 143 



substances, along with a number of others which cannot 

 be given here. 



Another achievement of Scheele's was the recognition of 

 heat radiation as a special form of the propagation of heat, 

 different from the well-known conduction of heat, such as 

 takes place in metals, and from the transference of heat by 

 convection, as in liquids, which was already recognised by 

 Black. The fact that concave mirrors and lenses collect the 

 heat of the sun's rays along with its light at their foci, had 

 been known for a very long time; but the heat at the focus 

 was regarded as an effect of visible light, and it had not been 

 recognised that invisible rays also exist which produce 

 warmth, and in other respects also, behave like light. 

 Scheele made this clear by a series of simple observations and 

 experiments upon the fire of a furnace. The heat of the fire 

 is emitted by convection of the heated air in the chimney, 

 and also of that in the room; but it is also emitted in straight 

 lines, as can be proved by using screens, and this takes 

 place without the air through which it passes being warmed. 

 This 'radiant' heat can also be collected by means of a con- 

 cave mirror, and sulphur can thus be ignited fairly far away 

 from the fire. Here it is not the light of the fire that pro- 

 duces this effect; for the experiment succeeds equally well 

 when the coals are hot but not glowing, and it fails when a 

 glass plate is interposed between the fire and the concave 

 mirror, though this plate allows the light to pass through it. 

 But the plate becomes warm, obviously because, though it 

 does not absorb the light, it does absorb the radiant heat. 

 At a later date (in the year 1800) W. Herschel, the discoverer 

 of the planet Uranus, showed that the invisible heat rays 

 discovered by Scheele fall, when refracted by a prism, out- 

 side the visible spectrum, since they are less refracted than 

 red light. Later they were suitably named the infra-red 

 rays. 



Scheele also investigated the already known blackening of 



