172 DYEING 



putting a sample of the dyed goods in boiling water, holding in solution a determinato 

 quantity of alum, tartar, soap, and vinegar, &c. Dufay improve! that l>;irbarou.s tost. 

 He considered that fast-dyed cloth could bo recognised by resisting an exposure of 

 twelve hours to the sunshine of summer, and to the midnight dews, or of sixteen days 

 in winter. 



In trying the stability of dyes, we may offer the following rules : 



That every stuff should be exposed to the light and air ; if it bo intended to be 

 worn abroad, it should bo exposed also to the wind and rain ; that carpets, moreover, 

 should be subject to friction and pulling, to prove their tenacity; and that cloths 

 to be washed should bo exposed to the action of hot water and soap. However, such 

 tests are not at all applicable to most of the colours dyed upon cotton stuff. Not 

 many of them can stand the action of hot water and soap, or even such acids as the 

 juice of fruits. Indigo-blue, one of the most permanent dyes on cotton, yields its 

 intensity to every operation of washing, even in pure water. 



Delaval's observations on the nature of dyes may be thus summed up. In tran- 

 sparent coloured substances, the colouring substance does not reflect any light ; and 

 when, by intercepting the light which was transmitted, it is hindered from passing 

 through substances, they do not vary from their former colour to any other colour, 

 but become entirely black ; and he instances a considerable number of coloured 

 liquors, none of them endued with reflective powers, which, when seen by transmitted 

 light, appeared severally in their true colours ; but all of them, when seen by incident 

 light, appeared black; which is also the case with black cherries, black currants, black 

 berries, &c., the juices of which appeared red when spread on a white ground, or 

 otherwise viewed by transmitted instead of incident light ; and he concludes, that 

 bleached linen, &c. 'when dyed with vegetable colours, do not differ in their 

 manner of acting on the rays of light, from natural vegetable bodies ; both yielding 

 their colours by transmitting, through the transparent coloured matter, the light which 

 is reflected from the white ground : ' it being apparent, from different experiments, 

 'that no reflecting power resides in any of their components, except in their white 

 matter only,' and that ' transparent coloured substances, placed in situations by which 

 transmission of light through them is intercepted, exhibit no colour, but become 

 entirely black.' 



The art of dyeing, therefore (according to Mr. Delaval), ' consists principally in 

 covering white substances, from which light is strongly reflected, with transparent 

 coloured media, which, according to their several colours, transmit more or less 

 copiously the rays reflected from the white,' since ' the transparent media them- 

 selves reflect no light ; and it is evident that if they yielded their colours by reflect- 

 ing instead of transmitting the rays, the whiteness or colour of the ground on 

 which they are applied would not in anywise alter or affect the colours which they 

 exhibit.' 



But when any opaque basis is interposed, the reflection is doubtless made by it 

 rather than by the substance of the dyed wool, silk, &c., and more especially when 

 such basis consists of the white earth of alum, or the white oxide of tin ; which, by 

 their strong reflective powers, greatly augment the lustre of colours. There are, 

 moreover, some opaque colouring matters, particularly the acetous, and other solu- 

 tions of iron, used to stain linen, cotton, &c.. which must necessarily themselves 

 reflect instead of transmit the light by which their colours are made perceptible. 



The compound or mixed colours are such as result from the combination of two 

 differently-coloured dye-stuffs, or from dyeing stuffs with one colour, and then wit li 

 another. The simple colours of the dyer are red, yellow, blue, and black, with whk-h, 

 when skilfully blended, he can produce every variety of tint. Perhaps the dun or 

 fawn colour might be added to the above, as it is directly obtained from a great many 

 vegetable substances. 



1. Red with yellow, produces orange ; a colour, which upon wool is given usually 

 with the spent scarlet bath. To this shade may be referred flame colour, pomegranate, 

 capuchin, prawn, jonquil, cassis, chamois, cafe au lait, aurora, marigold, orange-peel, 

 wwrdores, cinnamon, gold, &c. Snuff, chestnut, musk, and other shades are produced 

 by substituting walnut peels or sumach for bright yellow. If a little blue bo added 

 to orange, an olive is obtained. The only direct orange dyes are aunotto, and sub- 

 chromate of lead. 



The latter is never used for dyeing orange upon silk and wool, while the former is 

 now never used for cotton. An orange with annotto is very fugitive, even upon the 

 animal fibre ; but much more so upon cotton. Subchromate of lead is produced upon 

 cotton by dyeing it first a deep chrome yellow by acetate of lead and bichromatr -if 

 potash, as already noticed, and then passing the stuff so dyed through a hot solution 

 of an alkali or lime, which changes the dye from the yellow chromato to the state of 

 subchromato, which is deep orange. See SILK and WOOL DYEINO, 



