172 



Br. Thomas Thomson on [March 



20. White Discharge upon Bronze. 



The cloth is first soaked in a solution of sulphate or 

 chloride of manganese, and dried. It is then passed through 

 a strong caustic alkali, by which the white hydrated 

 protoxide of manganese is precipitated on the cloth. This, 

 by exposure to the air gradually darkens, being converted 

 into sesquioxide ; and this change is further promoted by 

 the action of bichromate of potash. 



Protochloride of tin is the substance best adapted for 

 producing white figures upon this ground. By its means 

 chloride of manganese is produced, which is readily removed 

 by water ; while peroxide of tin either takes the place of the 

 manganese or may be also rendered soluble by employing a 

 free acid along with the solution of tin. 



21. White upon Blue. 



The blue vat is a solution of deoxidized indigo in lime- 

 water. To form it the indigo is ground to a fine paste 

 with water, and then mixed with sulphate of iron and an 



