On Improvements in Black Writing Ink: 233 



became insoluble ; this was removed by filtration, when, by the 

 addition of the sulphate of iron, a very perfect and durable ink 

 was produced. In the above three processes, I conceive that a 

 considerable part of the mucilage, the tan, and the extract, were 

 respectively removed from the infusion, while the greater part 

 of the gallic acid would be left in solution. 



The three causes of deterioration in ink, the moulding, the 

 precipitation of the black matter, and the loss of colour, as they 

 are distinct operations, so we may presume that they depend on 

 the operation of different proximate principles. It is probable 

 that the moulding more particularly depends on the mucilage, 

 and the precipitation on the extract, from the property which 

 extractive matter possesses of forming insoluble compounds with 

 metallic oxides. As to the operation of the tan, from its affinity 

 for metallic salts, we may conjecture that, in the first instance, 

 it forms a triple compound with the gallic acid and the iron ; 

 and that, in consequence of the decomposition of the tan, this 

 compound is afterwards destroyed. Owing to the difficulty, if 

 not impossibility, of entirely depriving the infusion of galls of 

 any one of its ingredients, without in some degree affecting the 

 others, I was not able to obtain any results which can be regard- 

 ed as decisive ; but the general result of my experiments favours 

 the above opinion, and leads me to conclude, that, in proportion 

 as ink consists merely of the gallate of iron, it is less liable to 

 decomposition, or to experience any kind of change. 



The experiments to which I have alluded above, consisted in 

 forming a standard infusion, by macerating the powder of galls 

 in five times its weight in water, and comparing this with other 

 infusions which had either been suffered to mould, from which 

 the tan had been abstracted by gelatine, or which had been kept 

 for some time at the boiling temperature ; and by adding to each 

 of these respectively, both the recent solution of the sulphate of 

 iron, and a solution of it which had been for some time exposed 

 to the atmosphere. The nature of the black compound pro- 

 duced was examined by putting portions of it into cylindrical 

 jars, and observing the changes which they experienced with re- 

 spect either to the formation of mould, the deposition of their 

 contents, or any change of colour. The fluids were also com- 

 pared by dropping portions of them upon white tissue paper, in 



JULY — SEPTEMBER 1830. d 



