ON THE ACTION OF LIGHT UPON DYED COLOURS. 245 



Silk Patterns. 



Most of the foregoing colours were also dyed on silk, and the patterns 

 were exposed to light along with those on wool. The relative fastness of the 

 various colours was, for the most part, the same as on wool, the difference? 

 observed being too unimportant to necessitate a special classification foj 

 silk. In Class IV., Yellow for wool A F (Cr.) proved to be much more 

 fugitive on silk, whereas Chrysamin E, and Gr, Titan Yellow E, and Y, 

 Oriol, Cresotin Yellow R and G, and Chrysophenin appeared to be 

 somewhat faster. In Class III. the same remark applies to Cotton 

 Yellow G. 



The Indian dye-stuff Kamala was an additional one applied to silk, and 

 found to belong to the fugitive class, being very little faster than Annatto. 



General Observations. 



The first thing which strikes one when examining these orange and 

 yellow patterns is the comparatively large number of satisfactorily per- 

 manent colours. 



In the more or less fugitive class are to be found all the basic colours; 

 all the nitro-phenols, with the exception of Palatine Orange, and all the 

 bright yellows derived from the natural colouring matters by means of 

 aluminium and tin mordants, with the exception of those obtained from 

 "Weld. Comparatively few azo colours are met with in this group. 



The marked alteration in colour from yellow to orange shown in the 

 case of Picric Acid has long been known, and is ascribed to a reducing 

 action of the light. The equally striking change from orange to brown, 

 shown by Aurantia, does not, however, seem to have been previously 

 recorded. 



By far the largest number of yellows, ranging from ' moderately fast ' 

 to ' very fast,' ai'e to be found among the azo colours. Specially important 

 are those in which salicylic acid is a constituent element, since not only does 

 this impai't to the colour the power of foi-ming more or less stable lakes with 

 chromium and aluminium mordants, but it appears frequently to give the 

 colours the quality of fastness to light, even when no mordant is applied. 

 It is a fact of some importance that the colours obtained with aluminium 

 are practically as fast as those fixed with chromium, since the first-named 

 mordant gives much brighter and purer yellows. The tin mordant, so useful 

 in tlie production of the most brilliant orange and yellow colours obtainable 

 from the natural colouring matters, seems, however, to be of httle or no 

 advantage in connection with most of these azo-moixlant-colours, no doubt 

 because they are susceptible to the reducing action of the mordant usually 

 employed for wool — viz. stannous chloride. 



Very interesting in point of fastness to light are the azoxy colours, and 

 although unfortunately apt to dye wool somewhat irregularly, giving 

 speckled-looking colours, they are admirably adapted for silk and 

 cotton. 



Another interesting little group is that which includes Tartrazin, a 

 colour not only noteworthy for its fastness to light, but also because of its 

 brilliancy and purity. 



The fastness of Alizarin Orange is worthy of special mention, for it is 

 probably greater even than that exhibited by most other colours of the 



