OILS AND FATS. 367 



512. "Writing-Ink. Common writing-ink is prepared from 

 nut-galls and ferrous sulphate. A solution of the tannic 

 acid is obtained by boiling the galls in water, and this is 

 mixed with a solution of ferrous sulphate. A feriws tan- 

 nate results, which makes a very pale solution ; but by ex- 

 posure to the air the tannate becomes more highly oxidized, 

 and thus changes to a feme tannate, which is of a very 

 dark color. It is desirable, for the permanency of the writ- 

 ing, that this change should take place in part in the fibres 

 of the paper, and not wholly in the ink before it is used. 

 The ink is, therefore, bottled, and thus shut in from the air 

 before the change is completed, so that when used in writ- 

 ing it may be rather pale, and become gradually dark on the 

 paper. To keep the tannate from settling gum is added to 

 the ink, and to prevent moulding oil of cloves or creosote is 

 introduced in small quantity. Corrosive sublimate is a very 

 effective preventive of moulding, but it is obviously danger- 

 ous to employ it with the ordinary careless habits of people 

 in using ink. It is from the action of tannin on ferrous sul- 

 phate and other salts of iron that it is used in dyeing. 



513. Oils and Pats. These substances are found widely 

 distributed in both the vegetable and the animal kingdoms, 

 and are constituted very much alike in both. In plants 

 they are generally contained in the investing membranes 

 of seeds or in the cellular substance of fruits. There is sel- 

 dom any fatty matter in leaves or in roots. The principal 

 vegetable oils and fats are as follows : Linseed-oil, which is 

 pressed out from flax-seed ; Olive-oil, from the pulp of the 

 fruit of the olive-tree; Palm-oil, a yellow fat, similar to 

 butter, from the fruit of a species of palm-tree; Castor-oil, 

 from the seeds of the castor-oil plant ; Butter of Cacao, 

 the tallow-like fat of the cacoa-nut, the cause of the fat 

 particles which rise on boiled chocolate; Hemp-oil, from 

 hemp-seed, etc. Oils can also be obtained from pumpkin- 



