Attraction. 13 



vitriol is dissolved. The iron which this salt 

 (green vitriol) contains, has a strong attraction 

 for the gall water; and when they are mixed 

 together they unite, and the mixture becomes 

 black ; in fact, is made into ink. But when the 

 phial, No. 3, which contains aqua fortis (the 

 nitric acid, as it is called by chemists), is poured 

 in, the iron, which has a stronger attraction for it 

 than for the galls, unites with it, and having left 

 the galls, the liquid is again clear. Again, the phial 

 No. 4, contains potass, formerly called salt of tar- 

 tar, or of wormwood. It is the vegetable alkali 

 of chemists. The aqua fortis, or nitric acid, has 

 a stronger attraction for this alkaline matter than 

 it has for the iron ; it therefore drops the iron, 

 which again unites with the matter of the galls, 

 and the fluid resumes its black complexion. 



You may amuse yourselves with the same ex- 

 periment in another way. If you write a few 

 words with common ink (which you now know 

 how to make) upon a thick paper, and let them 

 dry,' and then take some aqua fortis diluted or 

 weakened with water, and with a feather drop or 

 rub it upon the letters, the writing will totally 

 disappear. When this is dry, with another fea- 

 ther smear it over with some of the solution of 

 potass or salt of tartar, and the writing will be 

 restored. 



These several kinds of attractions which I have 

 now mentioned, philosophers have ranged under 

 five distinct heads. The^r^, that, I mean, of 



