With what substance does it form its most important compound?] _Magnesia_ is the remaining alkali of vegetable ashes. It is well known as a medicine, both in the form of calcined magnesia, and, when mixed with sulphuric acid, as epsom salts. Magnesia is necessary to nearly all plants, but too much of it is poisonous, and it should be used with much care, as many soils already contain a sufficient quantity. It is often found in limestone rocks (that class called _dolomites_), and the injurious effects of some kinds of lime, as well as the barrenness of soils made from dolomites, may be attributed entirely to the fact that they contain too much magnesia. ACIDS. PHOSPHORIC ACID. _Phosphoric acid._--This subject is one of the greatest interest to the farmer. Phosphoric acid is composed of phosphorus and oxygen. The end of a loco-foco match contains phosphorus, and when it is lighted it unites with the oxygen of the atmosphere and forms phosphoric acid; this constitutes the white smoke which is seen for a moment before the sulphur commences burning. Being an acid, this substance has the power of combining with any of the alkalies. Its most important compound is with lime. [Will soils, deficient in phosphate of lime, produce good crops? From what source do plants obtain their phosphorus?] _Phosphate of lime_ forms about 65 per cent. of the dry weight of the bones of all animals, and it is all derived from the soil through the medium of plants. As plants are intended as food for animals, nature has provided that they shall not attain their perfection without taking up a supply of phosphate of lime as well as of the other earthy matters; consequently, there are many soils which will not produce good crops, simply because they are deficient in phosphate of lime. It is one of the most important ingredients of manures, and its value is dependent on certain conditions which will be hereafter explained. Another use of phosphoric acid in the plant is to supply it with a small amount of _phosphorus_, which seems to be required in the formation of the seed. SULPHURIC ACID. [What is sulphuric acid composed of? What is plaster? What is silica? Why is it necessary to the growth of plants? What compounds does it form with alkalies?] _Sulphuric acid_ is important to vegetation and is often needed to render soils fertile. It is composed of sulphur and oxygen, and is made for manufacturing purposes, by burning sulphur. With lime it forms _sulphate of lime_, which is gypsum or 'plaster.' In this form it is often found in nature, and is generally used in agriculture. Other important methods for supplying sulphuric acid will be described hereafter. It gives _to_ the plant a small portion of _sulphur_, which is necessary to the formation of some of its parts. NEUTRALS. SILICA. [How can you prove its existence in corn stalks? What instance does Liebig give to show its existence in grass? How do we supply silicates? Why does grain lodge?