4. It roughens the smooth round particles of sandy soils, and prevents their compacting, as they are often liable to do. 5. It is also of use in killing certain kinds of insects, and, when artificially applied, in smoothing the bark of fruit trees. The source from which this and the other inorganic matters required are to be obtained, will be fully considered in the section on manures. SODA. [Where is soda found most largely? What is Glauber's salts? What is washing soda? What are some of the uses of lime?] _Soda_, one of the alkalies contained in the ashes of plants, is very much the same as potash in its agricultural character. Its uses are the same as those of potash--before enumerated. Soda exists very largely in nature, as it forms an important part of common salt, whether in the ocean or in those inland deposits known as rock salt. When combined with sulphuric acid it forms sulphate of soda or _Glauber's salts_. In combination with carbonic acid, as carbonate of soda, it forms the common washing soda of the shops. It is often necessary to render soils fertile. LIME. _Lime_ is in many ways important in agriculture: 1. It is a constituent of plants and animals. 2. It assists in the decomposition of vegetable matter in the soil. 3. It corrects the acidity[E] of sour soils. 4. As chloride or sulphate of lime it is a good absorbent of fertilizing gases. [How is caustic lime made? How much carbonic acid is thus liberated? How does man resemble Sinbad the sailor?] In nature it usually exists in the form of carbonate of lime: that is, as marble, limestone, and chalk--these all being of the same composition. In manufacturing caustic (or quick) lime, it is customary to burn the carbonate of lime in a kiln; by this means the carbonic acid is thrown off into the atmosphere and the lime remains in a pure or caustic state. A French chemist states that every cubic yard of limestone that is burned, throws off _ten thousand_ cubic yards of carbonic acid, which may be used by plants. This reminds us of the story of Sinbad the sailor, where we read of the immense _genie_ who came out of a very small box by the sea-shore, much to the surprise of Sinbad, who could not believe his eyes, until the _genie_ changed himself into a cloud of smoke and went into the box again. Sinbad fastened the lid, and the _genie_ must have remained there until the box was destroyed. Now man is very much like Sinbad, he lets the carbonic acid out from the limestone (when it expands and becomes a gas); and then he raises a crop, the leaves of which drink it in and pack the carbon away in a very small compass as vegetable matter. Here it must remain until the plant is destroyed, when it becomes carbonic acid again, and occupies just as much space as ever. The burning of limestone is a very prolific source of carbonic acid. MAGNESIA. [What do you know about magnesia? What is phosphoric acid composed of?