196 Domestic Science 



be attached to the top of the burner. Half fill each 

 can with water, and cover the top of each with a sheet 

 of cardboard carrying a thermometer inserted through 

 a cork in the centre of the sheet. See that the bulbs 

 of the thermometers are submerged to equal depths 

 in the water. Leave the apparatus at rest for two 

 minutes and then read each thermometer. Light the 

 burner, turning on both gas and air supplies to the 

 fullest extent. Record the readings of the thermo- 

 meters at intervals of about one minute, stirring the 

 water with the thermometer before taking each reading. 

 The rise of temperature registered by the instrument 

 in the can with the blackened bottom will be more 

 rapid than that recorded by the other. Continue the 

 observations till the water in each can has shown 

 a considerable gain in temperature. Remove the cans 

 from the hot plate and place them side by side in a 

 quiet place, free from draughts. They should be kept 

 out of contact with the surface of the bench or other 

 supporting medium by placing the triangles underneath 

 them as before. At intervals of a minute or two, read 

 the thermometers, until the water has nearly reached 

 the temperature of its surroundings. The water in 

 the bright can will show the less rapid fall in tempera- 

 ture, counting the fall from the time at which the hotter 

 water in the other reaches the temperature at which 

 the water in the bright can was when the first reading 

 during cooling was registered. 



Bodies heated in the manner above described 

 receive heat chiefly by radiation. The heat rays are 

 more readily absorbed by the dull black surface and 

 become sensible as heat. The bright surface, however, 

 reflects a large proportion of the rays, just as a mirror 

 reflects light rays, and a lesser proportion of the rays 



