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compounds in nature: it is in this state, there-fore, that I shall describe their characteristic pro-perties.

1. Silica, or the earth of flints, in its pure andcrystallized form, is the substance known by thename of rock crystal, or Cornish diamond. As itis procured by chemists, it appears in the form of awhite impalpable powder. It is not soluble in thecommon acids, but dissolves by heat in fixed alka-line lixivia. It is an incombustible substance, forit is saturated with oxygene. I have proved it tobe a compound of oxygene, and the peculiar com-bustible body which I have named silicum; andfrom the experiments of Berzelius , it is probablethat it contains nearly equal weights of these twoelements.

2. These sensible properties of lime are wellknown. It exists in soils usually united to carbon-ic acid; which is easily disengaged from it by theattraction of the common acids. It is sometimesfound combined with the phosphoric and sulphuricacids. Its chemical properties and agencies in itspure state will be described in the Lecture on ma-nures obtained from the mineral kingdom. It issoluble in nitric and muriatic acids, and forms asubstance with sulphuric acid, difficult of solution,called gypsum. It is not soluble in alkaline solu-tions. It consists of one proportion 40 of the pe-culiar metallic substance, which I have named cal-cium; and one proportion 15 of oxygene.