182 LECTURES TO SCIENCE TEACHERS. 



which radiate most heat ; but if, previously to breathing 

 upon the glass, we slowly heat it so as to raise it to the 

 temperature of the body, then when we breathe upon 

 it we do not cool the breath below the temperature of 

 saturation, and we shall find that it can no longer 

 condense the vapour which is contained in the breath. 

 The glass is absolutely clear and transparent after being 

 breathed upon. 



Simultaneously with these advances in the science of 

 radiant heat, some very curious researches were being made 

 which tended to divide heat into different kinds. Sir 

 William Herschel had observed that the temperature of the 

 spectrum varies in different parts. He formed a spectrum 

 with a prism of glass, and, passing a thermometer along 

 the different parts of the spectrum, he found the tem- 

 perature increase as he approached the red end, and not 

 only so, but he got a very large amount of heat in the 

 part which was utterly invisible beyond the red. This 

 was a very important advance, and was taken up after- 

 wards by other celebrated experimenters by Seebeck, the 

 illustrious discoverer of thermo-electricity, who experi- 

 mented on light and other subjects. He found that the 

 position of the maximum point depended on the nature of 

 the prism which was employed ; thus when he employed a 

 prism of flint glass he found the maximum was beyond the 

 red, but when he employed one of water he found that the 

 maximum was in the yellow. The next person who threw 

 light on this subject was Delaroche. He employed a 

 luminous source of heat, and measured the effect which it 

 had upon the thermometer, but he found that as soon as 

 he placed a piece of glass between the luminous source of 

 heat and the thermometer, the thermometer indicated an 

 increase of temperature veiy much less ; that there was 

 only three or f our per cent, of heat transmitted through 

 the glass ; but when he employed heat which had already 

 passed through one piece of glass, then on putting another 

 piece of glass in front of it, he found that the temperature 

 was hardly diminished at all ; he found that heat, in fact, 

 which had once passed through a piece of glass could easily 

 pass through another piece of glass also. Thus there seemed 

 to be some reason for dividing heat into two kinds one 



