78 Colours and their Relations. [January, 



as these last have approximately among themselves, and 

 exhibit no indication of their being derived from one and 

 the same fundamental vibration with them. It appears, 

 therefore, safest to conclude that the approximate numerical 

 relation 2oH I = 27H 2 = 32H 4 is simply an indication that the 

 inertias of these three sets of atoms stand to each other 

 nearly in this relation. 



It seemed advisable to make these explanations in refer- 

 ence to the former essay on " Molecules, Ultimates, Atoms, 

 and Waves" with a view further to illustrate the subject of 

 which it treats. It is now proposed to consider more at 

 large the phenomena of colour simply as they present them- 

 selves to the eye, with their various relations. 



Colours may be divided into two great classes — intrinsic 

 and adventitious. Intrinsic colours depend on the arrange- 

 ment of the molecules, ultimates, or atoms constituting the 

 coloured substance ; while adventitious colours depend on 

 the disposition of aggregations of these into grains, fibres, 

 layers, prisms ; or they are due to the interference of wave 

 with wave, the superposition of wave upon wave, the separa- 

 tion of wave from wave of the luminiferous ether ; also in 

 some cases to an alteration in the rate of vibration of the 

 ethereal waves. 



Intrinsic colours first demand attention, the phenomena 

 which they present being comparatively few and simple. 

 In the case of an elementary substance, which, while in the 

 state of gas or vapour, exhibits colour, such as chlorine gas 

 and the vapours of iodine and bromine, the colour most 

 probably depends on the arrangement of the atoms con- 

 stituting the ultimates of those elements. And this phe- 

 nomenon furnishes a strong argument in favour of the view, 

 that the chemical elements are really compounded of still 

 more simple atoms. Did the colours of those elemental 

 vapours depend on vibrations performed simply by their 

 ultimates, seeing the vibrations would, in that case, be all 

 of one rate, the tint produced would be one or other of the 

 pure unmixed colours of the spectrum. But this they are 

 not, consequently the vibrations causing them must be of 

 various rates ; nor does it appear possible to find any other 

 cause of such a variation of rate than that of their being 

 due to the compound nature of the ultimates — their con- 

 sisting of atoms which, when set in motion by the ethereal 

 waves, vibrate at different rates, producing a compound tint. 



When the chemical elements are not in the gaseous or 

 vaporous condition, their colour probably depends on the 

 arrangement of the ultimates and their rates of vibration, 



