Dr Stark on the Influence of Colour on Odours. 93 



of the texture or fineness of the substance employed. Wool, 

 though generally coarser in the fibre than cotton, has yet a 

 greater attraction for odours ; and silk more than wool. The 

 general conclusion would appear to be, that animal substances 

 have a greater attraction for odours than vegetable matters; 

 and that all these have their power much increased by their 

 greater darkness or intensity of colour. These experiments 

 seem also to establish, that the absorption of odours by coloured 

 substances is regulated by the same law which governs the ab- 

 sorption of light and heat. The analogy goes still farther, for in 

 other experiments which I made with a view to ascertain this 

 point, I invariably found, that the power of colour in radiating 

 or giving out odours, was in strict relation to the radiation of 

 heat in the same circumstances. My first experiments in this 

 branch were with differently coloured wools, inclosed for a cer- 

 tain time in a drawer along with assafoetida and camphor, and 

 afterwards exposed for a specific period to the action of the air. 

 Though one can easily judge by the sense of smell alone the 

 different intensities which these articles have acquired imme- 

 diately on being taken out of the drawer, yet, after exposure 

 for some time to the air, the difference of intensity is much 

 more difficult to be perceived. In general, it seemed to me that 

 the whole of the substances lost their sensible odour in nearly the 

 same space of time, though the odorous particles given out by 

 the black were, of course, much greater in quantity than in the 

 others. To demonstrate this, I took pieces of card, coloured 

 as before, black ^ dark blue, brown, orange-red, and white, and, 

 after having exposed them to the vapour of camphor, in the 

 usual manner, they were taken out of the vessel, weighed, and 

 exposed in the apartment for twenty-four hours. Upon care- 

 fully weighing the cards at the end of this period, it was found 

 that the black had lost one grain ; the blue nearly as much ; the 

 brown ^^ of a grain ; the red j^ ; and the white yo- In about 

 six hours afterwards, the black and blue had completely lost 

 their camphor ; the brown and 7'ed had the merest trace, inap- 

 preciable by a delicate balance ; while the white still retained 

 about 5^5 of a grain. 



In another experiment with cards coloured dark blue, dark 



