INK. 



415 



seme yellow prussiate of potash in water intone vessel, and some sulphate 

 of iron in another, adding a few drops of nitric acid to the sulphate of iron ; 

 now mix the two liquids, and a magnificent blue colour will appear, in the 

 form of a light sediment ; this is to be put upon a paper filter, and well 

 washed by pouring over it warm water, and allowing it to run through ; a 

 warm solution of oxalic acid should now be mixed 

 with it, and the Prussian blue will dissolve into a 

 bright blue ink. 



Red ink is made by boiling chips or raspings of 

 Brazil wood in vinegar, and adding a little alum and 

 gum ; it keeps well, and is of a good colour. A red 

 ink of more beautiful appearance, but not so durable, 

 may be made by dissolving a few grains of carmine in 

 two or three teaspoonfuls of spirit of hartshorn. 



Marking ink is made by dissolving nitrate of 

 ide> silver in water, and then adding some solution of 

 ammonia, a little gum water, and some Indian ink to colour it. Printers' 

 ink is made by grinding drying oil with lamp-black. 



The powdered gall-nut is an excellent test for iron in water. It will 

 turn violet if any iron be present. 



FORMIC ACID (CH 2 2 ) is the 

 caustic means of defence employed 

 by ants, hence the term formic. It 

 can be artificially prepared by dis- 

 tilling a mixture of sugar, binoxide 

 of manganese, and sulphuric acid. 

 On the skin it will raise blisters. 



LACTIC ACID (C 3 H 6 O 3 ) is pre- %' 

 sent in vegetable and animal sub- 

 stances. Sour whey contains it, 

 and the presence of the acid in 

 the whey accounts for its power of removing from table-linen stains. 

 When what is called " lactic fermentation " occurs, milk is said to be 

 " turned." 



Fig. 425. Drying rooms for hides. 



II. 



BASES. 



The definition of a base is not easy. We have described bases as 

 substances which, combining with acids, form salts, but the definition of a 

 base is as unsatisfactory as that of acid or salt. All vegetable bases contain 

 nitrogen, are usually very bitter, possess no smell or colour, and are insolu- 

 ble in water. They arc usually strong poisons, but very useful in medicine. 



