17 1 Ohfcrvatlons on MadiUrmg, 



louring fubftances, when long expofed to the atmofpheric 

 air, do not give colours of the fame inlenfity and the fame 

 brightnefs as before j either becaufe thcfe fublianccs abforb 

 the oxygen of the atmoiphere, or that they procure this radical 

 from the w^ler which lliey atfra(^l:, or wliich they naturally 

 contain as a conrtituent princijiie, and wliich is decom)ioied 

 by a flow and infenfible fermentation, Tiie expofure, on the 

 grafs, of cotton or linen dyetl' a dark madder red, might fup- 

 port the idea of a change to a reddilh yellow; for this dark 

 colour becomes clearer hut fainter by the expofure, and then 

 alfumes a more agreeable fliade of crimfon. 1 have (hown, 

 in a Uicmoir on indigo, inierted in the Jouriial de I'hyjique 

 for the year 17S8, that nitric acid changes this blue fecula 

 into a vellowidi fubftanee: a (jniilar change takes place by 

 expofing, on the meadow, the Hnne fecula fixed on any (luff 

 whatever; and the yellow rcfuliing in thefe two wavs is 

 more foluble in warm water than in the fame liquid when 

 cold. It however ajjpcars that tlie couibination of oxvgen is 

 not the only caul'e of the change of colours, fince curtains of 

 any fluff dyed or coloured any fliade whatever by vegetable or 

 animal fubftances, and expofed to the light, lofe their colour 

 entirely in the courfc of time on the fide expofed to the folar 

 ravs, while the oppofite fide retains it for a confiderable lime. 

 If the ravs of the fun then give more vigour to living bodies 

 of the animal and vegetable kinjidom by difcnsaaing from 

 the latter oxvgen gas, it appears that thev aft with deftruc- 

 tive influence on the fame bodies deprived of life, by decom- 

 pofing their conftiiuent principles. In all cafes it will be 

 proper to preferve the colouring ingredients in drv places 

 Iheltered fron) the light, which adls upon thefe bodies per- 

 haps only by dccompofing the conftituent aqueous part, the 

 oxvsien of which may join the carbon to form carbonic acid. 

 Refinons and oilv fubftances fliould be preferved in the fame 

 wav. Thefe conjeotures prove at leafl that the action of the 

 fun's rays, or of lioht, on thefe bodics^Jn general prefents a 

 vaft field lor interdting experiments to be undertaken. 



If in madderiiig brighter colours are obtained by carefully 

 regulatintr the heat, a ficrifice is made at the fame time of a 

 fmall portion of the colouring purls of the madder, which 

 cannot be entirely exhaulicd except bv then increafing the 

 beat to ebullition ; but as the colours thus obtained are de- 

 graded more or lefs in the ratio of the quantity of the mad- 

 der, the gail-nut or (liumac ufcd, this 'method niuft be 

 employed wiih caution, and principally for common eflcfts, 

 cither in reg;ird to cotton or linen. To avoid as much as 

 poflible the lo!s of madder after the maddering of good arti- 

 cles 



