710 Mr. James J. Dobbie [April 4, 



which it is attached to the uitrogen is yellow. The two forms have 

 totally distinct absorption spectra. When one of the forms passes 

 into the other nnder the influence of the appropriate reagent, the 

 amount of change is proportional to the quantity of reagent added. 

 It is possible, therefore, by taking photographs after the addition 

 of each successive quantity of reagent to trace the progress of the 

 change through all its phases, and to ascertain how much of each 

 form is present at any time. This is done by comparison with a 

 series of reference plates prepared by photographing mixtures in 

 various definite proportions of two derivatives of cotarnine which 

 possess the same spectra as the two parent forms. 



Incidentally I may mention that these changes afford a basis for 

 the measurement of the relative strength of alkalis and acids. For 

 example, the change from the yellow to the colourless form of cotar- 

 nine takes place under the influence of alkalis, and by comparing the 

 amount of change produced by equivalent quantities of different 

 alkahs we obtain a measure of their strength. 



The spectrographic method of investigation has been applied with 

 marked success to the study of the vegetable alkaloids. Many of 

 these substances, such as quinine, morpliine, strychnine, and atro- 

 pine, possess powerful and valuable physiological properties, and their 

 careful investigation is of the greatest importance to medical science. 

 The number of atoms present in the molecule of an alkaloid is 

 nearly always large, frequently as many as fifty or sixty. It is obvious, 

 therefore, that the task of determining their exact relation to one 

 another must be one of no little difficulty, and as a matter of fact 

 the elucidation of the structure of an alkaloid is one of the hardest 

 problems that the organic chemist can set himself. Any method, 

 therefore, which assists the chemical examination is of pecuHar value 

 in this branch of organic chemistry. 



In investigating an alkaloid or any organic compound, the first 

 thing that the chemist has to do is to determine the elements of 

 which it is composed, and the number of atoms of each of those 

 elements present in one of its molecules or ultimate particles ; in 

 a word, what is known as its molecular formula. This is a com- 

 paratively easy operation : the difficulty comes when he attempts to 

 determine the structure of the molecule, i.e. the exact manner in 

 which the atoms are arranged with respect to one another. Now it 

 may happen that two or more alkaloids have the same, or nearly the 

 same, composition. The question arises, are they constructed on 

 the same plan ? It is in attempting to answer this question that 

 the value of the spectroscope asserts itself. If on dissolving the 

 alkaloids in some diactinic solvent and photographing their absorp- 

 tion spectra, we find that the spectra are wholly different, we may 

 conclude that in spite of the similarity, or even identity in their 

 composition, the substances are not intimately connected with one 

 another. If, on the other hand, we find that the spectra are the 



