12 Dr. Thomas Thomson on [Jan. 



through a solution of bichromate of potash, those parts 

 which contain the oxide of lead are dyed a beautiful yellow. 

 Chloride of tin is capable also of removing peroxide of 

 iron from cloth, by reducing it to chloride, as it does the 

 sesquioxide of manganese. For this purpose it is sometimes 

 printed on a deep colour, composed of peroxide of iron and 

 quercitron yellow. The protochloride of iron is formed 

 and washed away, while the oxide of tin remaining, con- 

 stitutes a mordant for the quercitron. Thus the parts to 

 which the tin was applied become yellow. 



Protochloride of tin is also employed occasionally, to 

 discharge the orange, consisting of dichromate of lead 

 from the cloth. This it does by reducing the chromic 

 acid to protoxide. But as the green oxide of chromium 

 still continues fixed, the discharged parts do not assume a 

 good white colour. But this does not much affect the blue 

 and purple colours substituted for the orange, by mixing 

 the tin with Prussian-blue, or with logwood. 



When protochloride of tin is decomposed by carbonate 

 of soda, protoxide of tin is obtained. This protoxide is 

 used along with potash, to render indigo soluble. The 

 protoxide deoxidizes the indigo, and the potash dissolve s 

 the yellow base. It is then applied to the cloth in the way 

 that will be explained afterwards. 



III. RESIST PASTES. 



These are substances which have the property of restor- 

 ing the blue colour to dissolved indigo, and thus, of pre- 

 venting it from becoming fixed on those parts to which the 

 resist-pastes have been applied. Any substance which has 

 the property of readily parting with oxygen, answers this 

 purpose. Sulphate of copper, or any salt containing black 

 oxide of copper, when put into the indigo vat, instantly 

 revives the indigo, by communicating oxygen to it. The 

 hydrated black oxide of copper has the same effect, and so 

 have the sesquioxide and deutoxide of manganese. 



The calico-printer's indigo vat is a very deep large vessel 

 filled with water, into which indigo, sulphate of iron, and 

 an excess of lime are put. The lime decomposes the sul- 

 phate of iron, and the disengaged protoxide of iron coming 

 in contact with the indigo at the bottom of the vat, deprives 



