8 SCIENTIFIC AMUSEMENTS. 



solution of sulphate of iron, which develops a purple black 

 colouring matter resembling ink ; by repeating the process two 

 or three times, alternately boiling in logwood liquor and steeping 

 in solution of sulphate of iron, the cloth becomes a tolerably good 

 black. Now dry the cloth and print on it from a wooden block 

 any device you please, using as printing fluid a strong solution of 

 citric or oxalic acid thickened with gum ; wherever this is applied 

 the black will be discharged, so [that a white pattern on a black 

 ground will result (compare Expts. 139, 140). Silk handkerchiefs 

 are sometimes dyed and then spotted with nitric acid, which dis- 

 charges the dye and forms a yellow colour by acting on the silk. 



Expt. 293. To prepare Lakes of various Colours. The 

 attraction of cotton or other vegetable fibre for metallic com- 

 pounds is regarded by some rather as a physical phenomenon than 

 as an example of chemical combination, although this view is 

 open to some discussion ; but the union of dyestuffs with metallic 

 compounds appears to be wholly of the nature of a chemical change ; 

 moreover, the colour of the resulting combination varies with the 

 nature of the metal employed. When suitable metallic compounds 

 are brought into contact with solutions of dyestuffs without the 

 presence of vegetable fibres, they absorb the colouring matters and 

 develop tinted solid matters, which, when collected and dried, can 

 be used as pigments ; such compounds are generally called lakes. 



Dissolve some alum in water, and divide the solution into 

 several portions, to each of which add a little solution of dyestuff 

 of various kinds. Now add a little diluted ammonia solution to 

 each ; this will act upon the alum, precipitating liydrated alumina 

 (oxide of aluminium combined with water), so that if no dyestuff 

 be present a white gelatinous precipitate will be thrown down ; 

 but if any colouring matter be present it is absorbed by the 

 alumina forming a coloured lake, the shade of which depends on 

 the proportion between the alumina and the quantity of colouring 

 matter taken up, whilst the tint varies with the nature of the 

 dyestuff used. Crimson Lake, Madder Lake, and a large variety 

 of other lakes made from cochineal, madder, and other colouring 

 matters, natural or artificial, are prepared in large quantities by 

 processes essentially identical with this in principle for the use of 

 artists, house decorators, wall paper manufacturers, and such like 

 users of pigments ; by using other metallic compounds in place of 

 or in combination with alum, the tints and shades of the resulting 

 lakes can be modified to almost any required extent. 



Expt. 294. To prepare Ink. Writing fluids of various kinds 

 are in use at the present day, many of which are simply solutions 

 of certain colouring matters thickened with gum, &c., so that the 



