140 On the Colour of Chemical Compounds. 



temperature means a greater amplitude of vibration, but not 

 a greater period. In fact, if the vibration be not quite harmonic 

 a greater amplitude may, as in the pendulum, require a longer 

 period. In addition to this a rise of temperature weakens 

 the cohesion between the molecules, and hence lessens the 

 force of restitution, thus causing the molecule to vibrate more 

 slowly, and consequently producing the same changes of 

 colour as are observed when the mass of the molecule is 

 increased. (For Ackroyd's explanation of the colour-changes 

 produced by heat see ' Chemical News/ xxxiv. p. 76.) 



The above theory appears to account fully for the otherwise 

 paradoxical fact, that either increase in molecular weight or 

 increase in temperature produces identically the same series 

 of colour-changes. 



The phenomenon referred to under II. (viz., that in any 

 binary compound an increase in the quantity of the electro- 

 negative element produces a colour-change towards the red 

 end of the spectrum) is probably due to the increase in the 

 molecular weight caused by the greater quantity of the 

 electro-negative element which the compound contains, and is 

 hence also fully accounted for by the above theory ; though, 

 for this explanation to hold good throughout, it would be 

 necessary in some cases, as in those of the oxides of Cu, Pb, 

 and Cr, to double or treble the generally received formula of 

 some of these oxides, thus : — 



Cu 2 (red), Cu 2 2 (black) ; Pb 3 3 (yellow), Pb 3 4 (red), 

 Pb 3 6 (brown) ; Cr 2 3 (green), Cr 2 6 (red). 



Finally, there is also some indication of the colour of com- 

 pounds being a periodic function of the atomic weight. This 

 is best seen in the case of the normal iodides. If a curve be 

 constructed in which the ordinates represent the atomic 

 weights of the positive element, and the abscissae a chromatic 

 scale rising from black through brown, red, orange, yellow, 

 green, &c, to white, then the curve so obtained will be of a 

 nature similar to Lothar Meyer's well-known curve of the 

 elements. 



In a subsequent paper I hope to apply the above facts to 

 the colours of organic compounds, whereby I believe it will 

 be possible to trace a connexion between the composition and 

 the colour of many dye-stuffs. 



