706 



Mr. James J. Dobbie 



[April 4, 



concentration, and a curve is drawn connecting the concentration 

 and the absorption as measured with reference to the lines of the metal 

 employed as a source of light. The values of the spectral lines are 

 usually stated in wave-lengths, but in these curves it is preferable to 

 express them in reciprocals of wave-lengths. It is also convenient 

 io make use of the logarithms of the numbers representing the 

 thicknesses of the layers examined, rather than the numbers them- 

 selves (Fig. 4.) 



If we now enquire whether the substances which affect light in 

 one or other of the different ways already indicated have themselves 

 anything in common, we find that it is with those which possess the 



Pig. 4. — Salicylic acid 0-001 normal solution. 



structure characteristic of benzene and its derivatives that the power 

 of absorbing the rays of particular parts of the spectrum is most fre- 

 quently, although not exclusively, associated 



Organic compounds, or compounds containing the element carbon, 

 are divided into fatty or aliphatic, in which the carbon atoms are 

 united in an open chain, and cyclic, in which the carbon atoms 

 form a closed chain or ring. Hexane, which is a constituent of 

 liquid paraffin, may be taken as an example of the first class. This 

 substance possesses six carbon atoms united together in a single chain 

 and having attached to them fourteen hydrogen atoms, giving as the 

 formula of the compound CgHi^. This substance is highly diactinic 

 or transparent to the ultra-violet rays, and nearly all compounds 



