110 LIGHT 



consequence, this subject, so interesting to the painter, the dyer, and the general 

 artist, has been allowed to remain uninvestigated. As soon, however, as these evi- 

 dences of a counterbalance of mutually opposing actions, in the elements of which the 

 solar light consists, offered themselves to view, it occurred to me, as a reasonable 

 subject of inquiry, whether this slow destruction of vegetable tints might not be due to 

 the feeble amount of residual action outstanding after imperfect mutual compensation, 

 in the ordinary way in which such colours are presented to light, i.e. to mixed rays. 

 It appeared therefore to merit inquiry, whether such colours, subjected to the un- 

 compensated action of the elementary rays of the spectrum, might not undergo 

 changes differing both in kind and in degree which mixed light produces on them, 

 and might not, moreover, by such changes indicate chemical properties in the rays 

 themselves hitherto unknown. 



' One of the most intense and beautiful of the vegetable blues is that yielded by the 

 blue petals of the dark velvety varieties of the common heartsease ( Viola tricolor). It 

 is best extracted by alcohol. The alcoholic tincture so obtained, after a few days keep- 

 ing in a stoppered phial, loses its fine blue colour, and changes to a pallid brownish 

 red, like that of port wine discoloured by age. 



' When spread on paper it hardly tinges it at first, and might be supposed to have 

 lost all colouring virtue, but that a few drops of very dilute sulphuric acid sprinkled 

 over it, indicate by the beautiful and intense rose colour developed where they fall, 

 the continued existence of the colouring principle. As the paper so moistened with 

 the tincture dries, however, the original blue colour begins to appear, and when quite 

 dry is full and rich. The tincture by long keeping loses this quality, and does not 

 seem capable of being restored. But the paper preserves its colour well, and is even 

 rather remarkable among vegetable colours for its permanence in the dark or in 

 common daylight. 



' A paper BO tinged of a very fine and full blue colour, was exposed to the solar 

 spectrum concentrated, as usual (October 11, 1839), by a prism and lens; a water- 

 prism, however, was used in the experiment, to command as large an area of sunbeam 

 as possible. The sun was poor and desultory ; nevertheless, in half an hour there 

 was an evident commencement of whitening from the fiducial yellow ray to the mean 

 red. In two hours and a half, the sunshine continuing very much interrupted by 

 clouds, the effect was marked by a considerable white patch extending from the 

 extreme red to the end of the violet ray, but not traceable beyond that limit. Its com- 

 mencement and termination were, however, very feeble, graduating off insensibly ; 

 but at the maximum, which occurred a little below the fiducial point (corresponding 

 nearly with the orange rays of the luminous spectrum), the blue colour was completely 

 discharged. Beyond the violet there was no indication of increase of colour, or of any 

 other action. I do not find that this paper is discoloured by mere radiant heat 

 unaccompanied with light.' 



The late Dr. George Wilson of Edinburgh made some exceedingly interesting experi- 

 ments on the influence of sun light over the action of dry gases on organic colours. The 

 results arrived at were communicated to the British Association, and an abstract of 

 the communication is published in their Transactions. The experiments were on 

 chlorine, sulphurous acid, sulphuretted hydrogen, carbonic acid, and a mixture of 

 sulphurous and carbonic acid, oxygen, hydrogen and nitrogen on organic colouring 

 matters. ' I had ascertained," says Dr. George Wilson, ' the action of the gases 

 mentioned already on vegetable colouring matters, so arranged, that both colouring 

 matter and gas should be as dry as possible, the aim of the inquiry being to elucidate 

 the theory of bleaching, by accounting for the action of dry chlorine upon dry colours. 

 In the course of this inquiry, I ascertained that in darkness dry chlorine may be kept 

 for three years in contact with colours without bleaching them, although when moist 

 it destroys their tints in a few seconds (see BLEACHING) ; and I thought it desirable 

 to ascertain whether dry chlorine was equally powerless as a bleacher when assisted 

 by sunlight. The general result of the inquiry was, that a few weeks sufficed for the 

 bleaching of a body by chlorine in sunlight, where months, I may even say years, 

 would not avail in darkness.' The form of the experiment was as follows : Four 

 tubes were connected together so as to form a continuous canal, through which a 

 current of gas could be sent. Each tube contained a small glass rod, on which seven 

 pieces of differently-coloured papers were spiked. It is not necessary hero to state 

 the colours employed, suffice it to say, that all the tubes thus contained seven different 

 coloured papers, of different origins, and easily distinguishable by the eye. They 

 were arranged in the same order in each tube, and were prepared as nearly as 

 possible of the same shade. These papers were carefully deprived of every trace of 

 moisture by a current of very dry air. The tubes were then filled with the gas, also 

 dried, on which the experiment was to be made. One tube of each series was kept 



