Inorganic bodies, as already shown, unite _in pairs_,--two elements join to form a binary compound; two binary compounds produce a salt; two salts associated together form a double salt. With organic bodies however the arrangement is different,--the elementary atoms are all grouped equally in one compound atom, which is highly complex in structure, and cannot be split up into binary products. Observe also, as characteristic of Organic Chemistry, the apparent similarity in composition between bodies which differ widely in properties. As examples take _Lignine_, or cotton fibre, and Starch,--each of which contains the three elements united as C{24}H{20}O{20}. _Mode of distinguishing between Organic and Inorganic matter._--A simple means of doing this is as follows:-- place the suspected substance upon a piece of Platinum-foil, and heat it to redness with a spirit-lamp: if it first _blackens_, and then burns completely away, it is probably of organic origin. This test depends upon the fact, that the constituent elements of organic bodies are all either themselves volatile, or capable of forming volatile combinations with Oxygen. Inorganic substances, on the other hand, are often unaffected by heat, or, if volatile, are dissipated without previous charring. The action of heat upon organic matter may further be illustrated by the combustion of coal or wood in an ordinary furnace;--first, an escape of Carbon and Hydrogen, united in the form of volatile gaseous matter, takes place, leaving behind a black cinder, which consists of Carbon and inorganic matter combined; afterwards this Carbon burns away into Carbonic Acid, and a grey ash is left which is composed of inorganic salts, and is indestructible by heat. CHAPTER II. VOCABULARY OF PHOTOGRAPHIC CHEMICALS. ACETIC ACID. Symbol, C{4}H{3}O{3} + HO. Atomic weight, 60. Acetic Acid is a product of the _oxidation_ of Alcohol. Spirituous liquids, when perfectly pure, are not affected by exposure to air; but if a portion of yeast, or Nitrogenous organic matter of any kind, be added, it soon acts as a _ferment_, and causes the spirit to unite with oxygen derived from the atmosphere, and so to become sour from formation of Acetic Acid, or "vinegar." Acetic Acid is also produced on a large scale by heating _wood_ in close vessels: a substance distils over which is Acetic Acid contaminated with empyreumatic and tarry matter; it is termed Pyroligneous Acid, and is much used in commerce. The most concentrated Acetic Acid may be obtained by neutralizing common vinegar with Carbonate of Soda, and crystallizing out the Acetate of Soda so formed; this Acetate of Soda is then distilled with Sulphuric Acid, which removes the Soda and liberates Acetic Acid: the Acetic Acid being volatile, distils over, and may be condensed.