476 



NATURE 



[September 17, 1903 



of vibration means a louder note ; in the case of molecules 

 greater intensity of absorption may be caused by a greater 

 amplitude of vibration in the molecules of the absorbing 

 medium, the number of molecules being constant. But by 

 greater amplitude it is not .to be understood that the rate 

 of vibration is increased. 



If this be so then, as the absorption intensity of anthracene 

 and naphthalene is, molecule for molecule, greater than 

 that of benzene, the amplitude of vibration of the molecules 

 of these substances is greater, but the rate of vibration is 

 slower. 



From the foregoing it will be observed that where \ is 

 the wave-length i/A. is the inverse wave-length, omitting 

 the correction for the refraction of air which is a very 

 small value, it is the oscillation frequency of the ether in 

 a small unit of time, and the most convenient measurement 

 for use in describing spectra. Seven years after the publica- 

 tion of these views Gerard Kriiss (1888) dealt with the 

 subject of coloured substances in a similar manner. From 

 the undulatory theory of light, deductions may be drawn 

 regarding the inner molecular movements or inter-atomic 

 movements within the molecules, inasmuch as, the vibra- 

 tions of the ether, which fills the intra-molecular space, 

 are a resultant within that space of the velocity and ampli- 

 tude of the molecular vibrations. 



Thus, if \ be the wave-length of a ray emitted by a 

 substance, and v the velocity of light, the number of vibra- 

 tions, n, which a molecule sends forth by movements of it 

 as a whole and of its parts can be determined by the equation 

 n — vjX. 



G. Kriiss made a series of calculations for coloured sub- 

 stances similar to those which I had made for colourless 

 substances and for ozone. 



Curves of Molecular Vibrations. 



Observations on absorption spectra should, whenever it 

 is possible, be made with reference to the quantity of sub- 

 ■stance which produces a given measurable effect. A 

 molecular weight in milligrams or a milligram-molecule is 

 a convenient quantity which may be dissolved in 20 c.c, 

 40 c.c, or lOQ c.c. of any non-absorbent liquid, and observed 

 through thicknesses of the solution varying from 25 mm. 

 to I mm. in thickness. When a series of photographs has 

 been measured a curve is plotted, which shows the general 

 and the selective absorption of the substance. The oscilla- 

 tion frequencies of the absorbed rays are taken as abscissze, 

 and the proportional thickness in millimetres of the weakest 

 of a series of solutions as ordinates. The curves are as 

 often as possible made continuous, and they are called 

 curves of molecular vibrations. 



The curves of the molecular vibrations present very 

 striking features : they are valuable physical constants 

 which enable one to classify and identify substances. 



Position Isomerism. 



Isomerides of the ortho-, meta-, and /lara-positions in 

 aromatic substances yield spectra with the absorption bands, 

 differing in position, in width, and in intensity. There 

 js no distinguishing character to be observed in the different 

 isomerides. Isomerism in the pyridine, quinoline, and 

 naphthalene derivatives has not yet been completely studied. 

 In such cases as have already passed under review there 

 is nothing that indicates the positions of the substituted 

 hydrogens. 



Stereo-isomerism. 



Where isomerism is not due to di'fferences in structure, 

 but simply to the distribution of the atoms in space, we have 

 no means of distinguishing isomeric substances from an 

 examination of their spectra ; for instance, benz-syn- 

 aldoxime and benz-an<i-aldoxime yield curves of molecular 

 vibrations which are identical. 



Tautomerism. 



The possibility, of an atom of hydrogen occupying alter- 

 native positions in a compound 



(NH-C:0:|:N :C-OH) 



so that it may be united to an atom of nitrogen or of 

 carbon in one instance, or to an atom of oxygen in another, 



NO. 1768, VOL. 68] 



easily gives rise to substances with different characters, 

 the one that of a phenol, the other that of a ketone. One 

 interpretation of the facts observed which has been very 

 commonly received may be stated thus. Certain compounds 

 have in their constitution an atom of hydrogen of a " roving 

 disposition " which at one time will attach itself to an 

 atom of oxygen, or to an atom of nitrogen, and anon it 

 will forsake one of these and unite itself to an atom of 

 carbon. The consequence of this " instability of character " 

 is that when a derivative of the compound is being pre- 

 pared or sought for by a chemical process, which according 

 to all previous knowledge ought to yield it, the substance 

 brought forth is of a different class, but withal of the same 

 composition; it is, in fact, an isomeride. 



According to another theory, the two isomeric derivatives 

 of the parent substance are present in equal proportions in 

 a solution in a state of equilibrium, and upon crystallisation 

 one or other of these assumes the solid form. Taking those 

 cases where a substance has a constitution which it is 

 believed has been correctly ascertained by chemical re- 

 actions, and which yields two isomeric alkyl derivatives, it 

 becomes a question as to which of these the parent sub- 

 stance has directly given birth to. The evidence from 

 chemical reactions has in many cases failed to give a satis- 

 factory answer, but the curves of molecular vibrations of 

 such substances afford the desired information concerning 

 the relationship of their constitution to that of their re- 

 spective derivatives. 



Most convincing evidence has been afforded by observ- 

 ations on their spectra, that several of the parent substances 

 are really not what they seem to be. 



Thus, isatin and methyl pseudo-isatin yield curves which 

 are almost identical, the sole difference between them being 

 due to the substitution of the alkyl radical for hydrogen, 

 the nature of which difference might have been predicted. 



Clearly the parent substance and the pseudo-derivative 

 are of the same nature and constitution. 



Carbostyril and methyl-pseudo-carbostyril, o-oxycarbanil 

 and its ethyl ether, obtained by boiling with potash and 

 ethyl iodide, are also similarly related, and they possess the 

 ketonic or lactam structure. 



On the other hand methylisatin, carbostyril, and the other 

 ether of o-oxycarbanil yield curves which are essentially 

 different from the foregoing, and are enolic or of the lactim 

 type. Generally speaking, the ketonic are more stable than 

 the enolic forms. Dibenzoyl-methane is ketonic, and the 

 tautomeric substance oxybenzal-acetophenone is enolic, and 

 in this instance the enolic form is that with the greatest 

 stability. The two substances yield different curves, and 

 the gradual change of the less stable into the more stable 

 form can be traced by photographing the spectra of the 

 solutions at intervals. 



I'he ethyl esters of dibenzoyl succinic acid are of interest 

 in this connection. There are three isomers known out of 

 the thirteen which are possible, and the spectra of these 

 have been studied. Knorr has given three formulae for 

 what he designates the a, /3, and 7 esters. Of these there 

 are two, the j3 and 7 forms, which give identical absorption 

 curves : they are of the ketonic type, and structurally 

 identical, but configuratively different, being stereo- 

 isomerides. 



The curve of molecular vibrations of the a ester is quite 

 different from that common to the ;8 and 7 compounds. 

 The o compound is of the enolic type, and it changes spon- 

 taneously at ordinary temperatures into the ketonic, thus 

 showing that in this case also the latter is the more stable^ 

 The transition from the one form into the other was seen 

 to be in progress, and after an interval of only three hours 

 the absorption band of the enolic ester had almost entirely 

 disappeared. In three weeks the transformation had be- 

 come complete, as was shown by the molecular vibration 

 curve of the a ester being almost exactly coincident with 

 that of the j3 and 7 forms. 



Another interesting example is afforded by the study of 

 phloroglucinol, it being a substance with a constitution of 

 a somewhat doubtful character, for owing to the ambiguity 

 of its behaviour towards chemical reagents it is impossible 

 to arrive at a decision from chemical evidence whether the 

 oxygen atoms are present in enolic or ketonic groups. 

 Towards some substances it behaves as a phenol, towards 

 others as a ketone. The doubt also presented itself as to 



