Acid, a compound of hydrogen, in which that element is united to an electro-negative radical. In common language the term is equivalent to the Latin word acichis, meaning anything sour. Oxygen was formerly considered to be the element upon which the existence of the acid character mainly depended, as its name (signifying generator of acids) implies; but later researches have brought to light a number of compounds containing hydrogen possessed of acid properties in which oxygen is not present. Hence hydrogen is now regarded as more truly the generator of acids than oxygen. The usual test for the presence of an acid is its property of changing blue vegetable colors to red. We are already acquainted with several hundred acids, most of them belonging to the organic kingdom, and new ones are constantly discovered by chemists. The juices of plants and the constituents of animal bodies furnish their peculiar acids; and with the changes these undergo new acids are generated by different modes of combination, which processes are now imitated by art so as to reproduce by synthesis a number of organic acids. Some acids, when uneom-bined, are gaseous, others fluid, and others solid.

Their properties also are as various as the conditions in which they exist.