May 8, 19 1 3] 



NATURE 



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photographic plate is sei at such an angle as to bring 

 all the rays emanating from the source of light into 

 focus at its surface afler they have passed through 

 the resolving prism, and for this purpose it is neces- 

 sary that the plate should have a very slight curva- 

 ture. The prisms and lenses of the apparatus are 

 made of quart/, which, unlike glass, is readily per- 

 meable by the ultra-violel rays (Fig. 1). The source 

 of light usually employed is that obtained by sparking 

 one " of the metals, such as iron, or a com- 

 bination of metals, such as cadmium alloyed with 

 lead and tin. In using the apparatus a photograph 

 is first taken of the spectrum of the source of light. 

 A layer of the substance to be examined, which, if a 

 solid, must be dissolved in a suitable diactinie solvent, 

 such as alcohol or water, is then interposed between 

 the source of light and the slit of the collimator, and 



the absorption bands, but their degree of persistence, 

 i.e. the range of concentration within which they are 

 exhibited, it is necessary, therefore, to varv the con- 

 centration of the solution or the thickness of the layer 

 si) .is tn cover the whole phenomena of absorption. 

 This is done by simply diluting the solution, 'or 

 diminishing the thickness of layer, on one hand, until 

 the entire spectrum is transmitted, and on the other 

 bv increasing the concentration or the thickness of 

 the layer until no further characteristic absorptive 

 effect is produced. Photographs are taken at each 

 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 (Fig. 3). 



If we now inquire whether the substances which 

 affect tight in one or other of the different wavs 



r lG. 2. -1. Spark spectrum of nickel and iron. 2 and 3. The sail 

 Alcoholic solutions of (4) pinene, (5) thiophen, (6) citric acid illu 

 acid, (10) quiiine hydrochloride illustrate selective absorption. 



light has passed through wa 

 at absorption, and alcoholic si 



:ane sugar respectively. 

 , (8) phenol (9) salicylic 



another photograph is taken. By comparison of the 

 two photographs it is seen what effect, if any, the 

 substance has had upon the transmission of the light. 



When organic substances are examined in this way 

 it is found that some allow light to pass freely through 

 them. Others shorten the spectrum by absorbing the 

 rays at the ultra-violet end to a greater or less extent, 

 and are said to show general absorption. Others, 

 again, possess the remarkable property of absorbing 

 rays of a particular wave-length, thereby producing 

 gaps or bands in the spectrum ; these are said to show 

 selective absorption (Fig. 2). 



In studying these phenomena in their relation to 

 the chemical characters of a substance, it is of im- 

 portance to determine not only the extent of the 

 general absorption and the number and position of 

 NO. 2271, VOL. 91] 



already indicated have themselves anything in com- 

 mon, we find that it is with those which possess the 

 structure characteristic of benzene and its derivatives 

 that the power of absorbing the rays of particular 

 parts of the spectrum is most frequently, 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 the formulaCjHj,. 

 It is highly diactinie or transparent to the ultra-violet 

 rays, and nearly all compounds belonging to the same 

 division of organic chemistry, such as alcohol? sugars, 



