Of a useful earth, which absorbs moisture and carbonic acid, and exists as limestone, or in marble and chalk, which, when burnt, become lime: in its native state it is called carbonate of lime, and is burnt to disengage the carbonic acid; when made into a paste, with one part water and three parts lime,[13] and mixed with some other mineral or metallic substances, it forms plastic cements and mortars; and afterwards, imbibing carbonic acid from the atmosphere, it becomes again carbonate of lime, as hard as at first; and hence its use in building. [Footnote 13: See Chapter XVI., article Lime.] _Plastic_, yielding, capable of being spread out or moulded. What do you mean by Carbon? A simple substance, whose most common form is purified charcoal: it is, in fact, the base of charcoal, divested of all impurities; combined with oxygen, it forms _carbonic acid_ gas, formerly called fixed air. It is diffused through all animal and vegetable bodies; and may be obtained by exposing them to a red heat. In its pure, crystallized state, it constitutes the diamond, and as graphite, is used in making the so-called lead-pencils.[14] [Footnote 14: See Chapter XIV., article Diamond.] What is Oxygen? Air, mentioned in the first chapter of this work as the gaseous substance which composes the atmosphere, is formed by a mixture of two distinct elements, one called Nitrogen, or Azote, the other Oxygen. Oxygen is, therefore, an element or simple substance diffused generally through nature, and its different combinations are essential to animal life and combustion. It is, in fact, the most active agent in nature, and the principle of acidity and combustion. So wholesome and necessary is oxygen to life, that it is often called vital air. _Agent_, an actor; a person or thing possessing the faculty of action. _Essential_, necessary. What are the properties of Nitrogen or Azote? Nitrogen is a substance also generally diffused through nature, and particularly in animal bodies, and causes great changes in those absorbing or exposed to it. This gas, combined with oxygen and hydrogen, produces neither light, heat, nor combustion, but serves to dilute the others: of itself, it is hurtful to animal life. Nitrogen makes the principal part of the salt we call _nitre_. What is meant by Combustion? The decomposition of bodies by the action of fire; the union of combustible bodies with the oxygen of the atmosphere. The greater access the air has to a burning body, the more rapid and complete is the process. _Combustible_, capable of taking fire. _Access_, the means or liberty of approach to anything. Are all bodies equally combustible?