I4f INTRODUCTION. 
SO. Nitric Acid is a compound of oxygen and azote, 
or nitrogen, in the proportion of twenty-five parts, by 
weight, of the latter to seventy-five of the former. It 
is one of the constituent parts of nitre, or saltpetre 
(20) ; and, in a pure state, is transparent and colour- 
less, like water. By the action of light, however, it 
soon becomes yellow; and, if exposed to the air, it 
emits yellow fumes, which even tinge the air of the 
same colour. To the taste it is extremely acid. It dyes 
the skin a yellow colour, which is very difficult to be 
removed ; and it is so corrosive as to destroy almost 
every substance into which it penetrates. If poured 
upon oils, it sets them on fire. With various bases it 
forms compounds called nitrats. This acid, which 
hitherto has never otherwise been obtained than mixed 
with water, is chiefly known in commerce by the name 
of aquafortis (206). 
EARTHS. 
31. The solid contents of the globe are composed of 
several elementary substances, amongst which have been 
enumerated no fewer than nine different kinds of earth: 
1. Silex. 4. Glucine. 7- Strontian. 
2. Alumine. 5. Yttria. 8. Lime. 
3. Zircon. 6. Barytes. 9. Magnesia. 
These, when freed from foreign admixture, are, for 
the most part, of white colour, not soluble in water, 
not combustible, and do not exceed four times the 
weight of water. 
32. The whole of these earths have, till lately, been 
considered simple and uncombined substances; but, by 
the discoveries of Sir Humphry Davy, it has been as- 
certained that four of them have a metallic basis, and 
are in fact metallic oxides, or compounds consisting of 
