carbonic oxide burning back again by taking more oxygen from the air above the coals. Carbonic oxide combines directly with two or three of the metals, as, for instance, it forms a volatile compound with nickel, at a certain temperature, and this is decomposed again at a higher temperature. The like takes place with iron, although in very small quantities, but the observation throws some light on the processes of reduction. Carbonic oxide is neither acid nor basic, but carbonic acid is an acid oxide, and as such unites with oxides of the basic class to form another range of compounds. Thus, for example, the oxide of the metal _calcium_ is quicklime, which is strongly basic, and this directly combines with carbonic acid, forming a _neutral_ substance called in systematic chemistry _calcium carbonate_, or more commonly but less correctly, carbonate of lime, familiar to everyone in the compact state as limestone, and marble, and in a more or less pulverent condition as chalk. When any of these is heated to redness, carbonic acid is expelled and quicklime remains. Like most oxides, quicklime forms a compound with water, the combination being attended with the extrication of much heat, the compact quicklime swelling and crumbling into _slaked lime_. The chemist’s term for a compound of a _basic_ oxide with water is _hydrate_, while that of an _acid_ oxide with water is for him properly an _acid_, or in order to particularly distinguish this class, an _oxy-acid_. It was however the older practice to give the name of acid to the oxide alone, and this naming having found its way into popular language is much more familiar to the non-scientific reader. The systematic names of the two compounds of carbon and oxygen are _carbon monoxide_ and _carbon dioxide_, but we shall use here the more familiar terms carbonic oxide and carbonic acid.