3i6 NINETEENTH CENTURY. pt. hi. 



give us the whole of the dispersed ray ; for there are many 

 invisible rays at both ends of the coloured part which are 

 very active, though we cannot see them. 



It had always been thought that the hottest rays must be 

 those, such as the yellow ones, which give the most light, and 

 in the year 1800 Sir William Herschel, wishing to try this, 

 took a thermometer and passed it gradually from one end 

 to the other of the coloured band. The result was curious. 

 He began at the violet end of the spectrum (Plate I. No. i, 

 p. 320), and, as he expected, the thermometer rose higher 

 and higher as he approached the yellow part ; but to his 

 surprise it did not stop here. When he passed on through 

 the yellow into the red, the heat still increased, and even 

 became more intense as he passed out of the coloured band 

 altogether into the darkness beyond. By this experiment he 

 found that the heat-rays extend for some distance beyond 

 the red colour, and that they are strongest in that part where 

 no light is to be seen. 



Discovery of Chemical Eays by Ritter, 1801. — Soon 

 after Sir William Herschel had discovered the dark heat- 

 rays, a still more remarkable fact was brought to light about 

 the violet end of the spectrum. The Danish chemist Scheele, 

 who you will remember as one of the discoverers of oxygen 

 (see p. 232), had once remarked that nitrate of silver will 

 turn black if the violet rays of a spectrum are thrown upon 

 it. In 1 80 1, Professor Ritter, of Jena, was repeating this 

 experiment, and he found that the black patches appeared 

 slightly on those parts of the paper where the violet rays 

 fell, but very strongly indeed beyond those rays where the 

 spectrum was quite dark. So that at this end also there are 

 invisible rays, and these have the extraordinary power of de- 

 composing or breaking up nitrate of silver, and some other 



