18 INTRODUCTION. 
country, but always in combination with some acid, 
carbonic (26), sulphuric (24<), boracic (28), fluoric (27), 
or phosphoric (25). This substance has a metallic basis, 
which has been denominated calcium. 
41. Magnesia is a light and perfectly white kind of 
earth, of soft powdery appearance, without taste or 
smell, and somewhat more than twice as heavy as water. 
Ifc is not found in this pure state in nature, but may be 
prepared from Epsom salt, which consists of magnesia 
in union with sulphuric acid (24<). The slightly acrid 
taste that is perceptible in the magnesia used in medi- 
cine arises from a portion of lime which it contains, 
This substance does not dissolve in water, but is solu- 
ble in every kind of acid. It has the property of chang- 
ing delicate blue colours to green, 
ALKALIES. 
42 Alkalies are substances which enter into the com- 
position of several kinds of minerals, and are 'known by 
their property of changing the colour of blue vegetable 
juices to green, and by a peculiarly acrid, caustic, and 
nauseous taste, which it is impossible to describe, but 
which, after it has been once experienced, will easily be 
recollected. Alkalies corrode and dissolve animal sub- 
stances, and unite with oil and fat in such manner as to 
form the well known compound called soap. They 
readily dissolve in water ; and, when mixed with acids r 
form what have been denominated neutral salts. 
4?3. The alkalies at present known are three in num- 
ber; potash (205), soda (200), and ammonia (207). Of 
these the two former, although till lately they have 
been considered simple substances, have been shown 
by Sir H. Davy to have metallic bases. 
