590 REPORT— 1903. 



It follows from this that the vibrations within the molecules are not independent, 

 hut are a consequence, of the fundamental molecular vibrations, like the harmonics 

 of a stretched string or of a bell. 



The term absorptive power has generally been used with respect to the extent 

 of ravs of the spectrum absorbed, but there is intensity of absorption to be con- 

 sidered. In the case of a vibrating string or tuning-fork greater amplitude 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 -^ is 



A 



the inverse wave-length, and, 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 publication 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 



vibrations of the ether, which fills the intra-molecular space, are a resultant within 



that space of the velocity and amplitude of the molecular vibrations. 



Thus, if X be the wave-length of a ray emitted by a substance, and v the velocity 



of light, the number of vibrations, n, which a molecule sends forth by movements 



of it as a whole and of its parts can by determined by the equation w= ~. 



\ 



G. Kriiss made a series of calculations for coloured substances 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 substance 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 100 c.c. of any non- 

 absorbent liquid, and observed through thicknesses of the solution varying from 

 25 mm. to 1 mm. in thickness. When a series of photographs have been 

 measured a curve is plotted, which shows the general and the selective absorption 

 of the substance. The oscillation frequencies of the absorbed rays are taken as 

 abscissae, 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 ortfio-, met a-, and /jara-positions in aromatic substances 

 yield spectra with the absorption bands, differing in position, in width, and in 

 intensity. There is no distinguishing character to be observed in the different 

 isomerides. Isomerism in the pyridine, quinoline, and naphthalene derivatives has- 

 not vet been completely studied. In such cases as have already passed under 

 review there is nothing that indicates the positions of the substituted hydrogens. 



iSfereo-isomerism. 



Where isomerism is not due to differences in structure, but simply to the 

 distribution of the atoms in space, we have no means of distinguishing isomeric 



