39. _Strontian_ is an earth which, like barytes, is not found otherwise than in combination with sulphuric and carbonic acids. It occurs in various parts of the world, and, when purified, forms a porous mass of greyish white colour, acrid taste, and somewhat alkaline nature. This earth converts vegetable blue colours to green, but does not act so strongly on animal bodies as barytes, nor is it poisonous, like that substance. 40. _Lime_, the basis of all those substances which are denominated _calcareous_, is only to be obtained in a state of purity by artificial process. Combined with carbonic acid (26) it forms limestone (140), chalk, and marble; all of which are capable of being converted into lime by burning. Lime may also be obtained from oyster and other sea shells. When pure, it is of white colour, and moderately hard substance, though it is easily reducible to powder. Its taste is burning and acrid; and, like the alkalies, it changes vegetable blue colours to green. It has likewise the property of corroding and destroying animal substances. Lime, when pure, absorbs water rapidly, becomes hot, and falls into powder. Even if exposed to the open air it gradually attracts moisture, and assumes a powdery form; soon after which it becomes saturated with carbonic acid (26) from the atmosphere, and is thereby again converted into carbonat of lime (140). It occurs abundantly in almost every 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. It 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 medicine arises from a portion of lime which it contains. This substance does not dissolve in water, but is soluble in every kind of acid. It has the property of changing delicate blue colours to green. ALKALIES. 42. Alkalies are substances which enter into the composition 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 substances, 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, form what have been denominated neutral salts.