52 



INFRA-RED TRANSMISSION SPECTRA. 



80% 



70 



60 



50 



C 



'</> 



.4-0 



E 



w 

 <rj30 



20 



10 



I.I 



1.3 



their high reflecting power, which is uniform throughout the infra-red. 

 Their emission (arc and spark) spectra consist of numerous fine lines, 

 which occur throughout the visible and ultra-violet part of the spectrum. 



No strong lines have yet been found in 

 the infra-red, except in the alkali metals. 

 On the other hand, the non-metals (insu- 

 lators) have absorption bands throughout 

 the spectrum. Their reflecting power is 

 low, and in some compounds is highly 

 selective in the infra-red. They have 

 emission lines which extend far into the 

 infra-red. The elements on the border 

 line between metals and non-metals are 

 of peculiar interest, and the reflecting 

 power of their compounds deserves further 

 study. A notable example is silicon, 

 which from the known properties of non- 

 metals would be expected to have a uni- 

 5 A formly low reflecting power throughout the 

 fig. 35- chlorides of Yttrium Group (a); infra-red; in the form of a compound, 



Neodymium nitrate. o-^> ,i n . o i 



SiG 2 , the reflecting power at 8.5 and 9 jj. 

 attains a value as high as that of metals. The elements iodine, selenium, 

 and sulphur have a uniformly low reflecting power. It would be inter- 

 esting to learn whether only the compounds of non-metals have bands 

 of selective reflection in the infra-red; all of our present data indicate 

 that only in a compound (cf. Si0 2 ) do the ions of non-metals attain a 

 freedom such as obtains in metals. 



Silver. 

 (Colloidal film, on glass. Curve b, fig. 36.) 



The film examined was made 1 by Prof. R. W. Wood, and was of a 

 deep ruby-red color. In fig. 36, curve b, is given the transmission curve 

 of a thin uniform film of colloidal silver, deposited on glass. The trans- 

 mission begins in the red, and increases uniformly throughout the infra-red 

 to 4 fi, beyond which it was not possible to extend the observations on 

 account of the opacity of the glass. This is exactly the reverse effect 

 observed with a metallic film (see fig. 24), where the transparency lies in 

 the violet and the opacity extends throughout the infra-red. The film of 

 colloidal silver behaves like a turbid medium, which has the property of 

 increasing in transparency with wave-length. This is in agreement with 

 Garnett, 2 who concludes from the optical properties of Cary Lea's so-called 

 solutions of allotropic silver that they consist of small spheres of silver, i.e., 



1 Cary Lea's Allotropic Silver, Amer. Jour. Sci., vol. 37, p. 746, 1889. 



2 Garnett: Proc. Roy. Soc. (A), 76, p. 370, 1905-06; Phil. Trans., A, pp. 385-420, 1904. 



