194 Domestic Science 



the heat. The interposition of a book, the hand, or 

 a fan, between the fire and the face is productive of 

 an immediate sensation of coolness. 



129. The name given to the method of trans- 

 mission of heat we are now considering is "radiation", 

 and we speak of the heat so transmitted as " radiant " 

 heat. Rays of light and of heat are of similar character, 

 some rays being able to give rise to both sensations. 

 A familiar experiment will emphasise this point. 



EXPERIMENT 72. Obtain a glass lens, such as is in 

 common use for magnifying small objects. Hold this 

 with one face turned towards the sun, and on the 

 other side of the lens place a sheet of brown paper 

 so that it lies in a plane parallel to that of the middle 

 of the lens. A spot of light will be seen on the surface 

 of the paper. This is an image of the sun, and, by 

 moving the paper towards or away from the lens, 

 this spot may be made to vary in size. Variation in 

 brightness will accompany the change in size, the 

 smaller the spot the brighter being its appearance. 

 The lens is constructed in such a way as to bend the 

 rays of light from their original paths, and to crowd 

 them together at some spot near the lens. This is 

 called " focusing " the rays. Heat rays are similarly 

 bent in their passage through the lens and evidence 

 of this may be afforded, if the lens be a good one, by 

 the scorching of the paper when the spot of light is 

 of least diameter. Should this not occur, place the 

 hand or the bulb of a thermometer so as to receive 

 this smallest image of the sun, when the increased 

 warmth at this spot will be readily shown. 



130. All substances upon which heat rays strike 

 are capable of receiving heat by this means but all 

 do not exhibit this quality to the same extent. 



