708 Mr. James J. Dobbie [April 4, 



group we obtain three distinct nitrophenols. The ultimate particles 

 or molecules of these nitrophenols are all composed of the same 

 elements carbon, hydrogen, oxygen and nitrogen, and of the same 

 number of atoms of each element. Such substances are said to be 

 isomeric, i.e. they are made up of equal parts, although they do not 

 possess the same properties. The difference between them lies in the 

 arrangement of the parts relatively to each other ; in this case in the 

 position of the nitro-group in relation to the hydroxyl-group. On 

 comparing the spectra of the three nitrophenols we find that they 

 differ in quite a marked manner from one another, and afford an 

 illustration of the important general rule that substances which have 

 the same composition give different spectra if they differ in struc- 

 ture, i.e. in the arrangement of the atoms of their molecules. 



It will have been noticed that the substitution of the nitro-group 

 for hydrogen in phenol has the effect of shifting the absorption band 

 nearer to the visible region. One of the three nitrophenols has a 

 yellow colour, and in this case the gap in the spectrum cuts a Httle 

 way into the violet end of the visible region. By the addition of 

 soda to the solution the colour is changed to red, and on examining 

 the spectrum of this solution we see that the gap now extends far into 

 the visible region. This example will serve to illustrate the close 

 connexion that exists between the study of absorption spectra and 

 the origin of colour, an interesting branch of the subject with which, 

 however, it is impossible for me to deal within the limits of this 

 discourse. 



In the nitrophenols we have an example of what is known as 

 structural isomerism, or position isomerism, because the phenomenon 

 depends upon differences in the position or arrangement of the atoms 

 within the molecule — in other words, on the internal structure of the 

 molecule. But it is possible to have two substances of the same 

 composition and structure not identical, but related to one an- 

 other as an object is to its mirror-image. Substances so related 

 are termed optical isomers or stereo-isomers. The spectra of isomers 

 of this class, unlike those of structural isomers, do not differ. This 

 leads to an important application of absorption spectra in chemical 

 investigations. If two substances have the same composition but 

 different spectra, we know that they must be structurally different ; 

 if, on the other hand, they have the same composition and the same 

 spectra, and yet are not identical, there is a strong probability, 

 although not a certainty, that they are optical isomers. To take 

 an example. We know three substances of vegetable origin — papa- 

 verine (from opium), canadine (from the roots of Hydrastis cana- 

 densis^, and tetra-hydro-berberine (a derivative of the principal 

 alkaloid obtained from species of the barberry) — which have the same 

 chemical composition represented by the formula C20H21NO4. Papa- 

 verine and canadine differ widely in their spectra and in their struc- 

 ture ; canadine and tetra-hydro-berberine have identical spectra, 



