chap. xxvi.] PROPERTIES OF THE SPECTRUM. 33$ 



gives a greater deflection when the pile is in the red 

 end than when it is in either the yellow or violet part. 

 As the result of various elaborate researches made 

 since Herschell's time, by Seebeck, Milloni, and Tyn- 

 dall, a large number of facts regarding the calorific 

 effects of the spectrum has been obtained. It is now 

 known that the greatest heating effect is not obtained 

 even in the red, but beyond it. There are rays, that 

 is to say, of less refrangibility than the red, outside 

 of the red and invisible to our eyes, whose heating 

 effects are greater than those of the red. The maxi- 

 mum heating effects are obtained by these ultra-red 

 rays as they are called. It was found that certain 

 substances had the property of absorbing some of the 

 heat rays, while others, and notably rock salt, per- 

 mitted the heat rays to pass, absorbing very little. 

 The property of transmitting heat rays is called 

 DIATHERMANCY, that of absorbing them ATHERMANCY. 

 Tynclall found that solutions of iodine refused to 

 transmit light rays, but were quite pervious to heat 

 rays. He therefore interposed in the path of an 

 electric beam, a globe containing a solution of iodine 

 in bisulphide of carbon. The light rays were all 

 retained, and no visible rays issued from the lamp. 

 Yet he was able to focus the invisible heat rays on 

 to a piece of carbon,, and render it red hot, and to 

 treat a piece of platinum in a similar way. The heat 

 rays were detected as far beyond the extreme red as 

 the whole length of the visible spectrum. 



The chemical effects of the spectrum were 

 proved by Scheele in 1777 to be specially intense at 

 the violet end, since chloride of silver blackened more 

 speedily in the violet than in any other part of the 

 spectrum. Hitter proved that in the invisible part of 

 the spectrum, beyond the violet, there existed chemi- 

 cally active rays. Beyond the violet there are rays 

 of greater refrangibility than the violet, vibrations of 



