ACl 



AGO 



the transmission of the image of any ob- 

 ject through such a medium, are de- 

 stroyed. The colours which appear round 

 the edges of an object, when viewed 

 through an ordinary telescope, are pro- 

 duced by the different refrangibility of 

 the rays of light ; and telescopes which 

 are constructed so as to counteract or 

 prevent this aberration, are termed achro- 

 matic. 



ACI'CULAR {acicula, a little needle). 

 A term applied, in crystallography, to 

 needle shaped crystals ; and, in botany, to 

 the leaves of plants which are long, stiff, 

 and pointed, like a needle ; or to surfaces 

 which are marked with fine needle-like 

 streaks. 



A'CIDS. A class of compounds which 

 generally possess a sharp and sour taste, 

 and are often highly corrosive ; they red- 

 den the infusions of blue vegetable co- 

 lours, and combine with the alkalies, 

 earths, and metallic oxides, forming 

 compounds in which the characters of 

 the constituents are entirely destroyed, 

 and new characters produced differing in 

 every respect from those previously ex- 

 isting. See Alkalies. 



1. Oxygen Acids. "When the same ele- 

 ment forms two acid compounds with 

 oxygen, the name of that which contains 

 the greater proportion of oxygen is made 

 to terminate in ic, the other in ous, as in 

 sulphuric and sulphurous acids. A lower 

 degree of oxidation is expressed by pre- 

 fixing the Greek preposition hypo {bno, 

 under), as in hypo-sulphurous acid; while 

 another new compound, intermediate be- 

 tween the sulphurous and sulphuric acids, 

 was named hypo-sulphuric acid. On the 

 same principle, the highest degree of oxi- 

 dation is expressed by prefixing the 

 Greek preposition hyper {vnep, over), as 

 in hyper-chloric acid. This nomenclature 

 has been adopted for all analogous acids. 



2. Hydrogen Acids. These are acid 

 compounds of certain substances, as 

 chlorine, sulphur, and cyanogen, with 

 hydrogen, and they are hence called hy- 

 dracids. In these compounds the names 

 of both constituents appear, as in the 

 terms hydrochloric, hydrosulphuric, and 

 hydrocyanic acid. Thenard has lately 

 altered these names to chlnrhydric, sul- 

 phohydric, and cyanhydric acid, which 

 are better terms. 



3. Sulphur acids. In this class of acids 

 sulphur is united with the other element 

 in the place of oxygen. The names of 

 the corresponding oxygen acids are some- 

 times applied to these with the prefix 



sulpha, as sulpho-arsenious and sulpho- 

 arsenic acids, which resemble arsenious 

 and arsenic acids respectively in com- 

 position, but contain sulphur instead of 

 oxygen. 



ACIDIFI'ABLE. Capable of being 

 converted into an acid by an acidifying 

 principle. Substances possessing this 

 property are called radicals, or acidifiable 

 bases. 



A'CIDIFYING PRINCIPLE. That 

 princijjle which is capable of converting 

 a substance into an acid. 



ACIDPMETRY {acidum, an acid, )iie- 

 Tpov, a measure). The process of mea- 

 suring the strength of an acid, by satura- 

 ting a given weight of it with an alkaline 

 base : the quantity of ihe base required 

 for saturation is a measure of the strength 

 of the acid. 



ACIDULOUS. Slightly acid; a term 

 frequently applied to mineral waters 

 which contain carbonic acid ; and, in 

 chemical language, to those salts in which 

 the base is combined with such an excess 

 of acid, that they distinctly exhibit acid 

 properties, as the super-tartrate of po- 

 tassa. 



ACINA'CIFORM [acinaces, a scimitar, 

 forma, likeness). Scimitar-shaped ; plane 

 on the sides, with one border thick, the 

 other thin, as the succulent leaves of the 

 Mesembryanthemum acinaciforme. 



A'CINI (plural of acinus, a grape- 

 stone). A term applied by some carpo- 

 logists to the minute component parts of 

 the raspberry : incorrectly, however ; for 

 these are drupes. The term is also ap- 

 plied to the secerning parts of glands, 

 when they are suspended like grains or 

 small berries from a slender stem. 



ACLI'NIC LINE (a, priv., kXIvco, to 

 incline). The name given by Professor 

 August to an irregular curve in the 

 neighbourhood of the terrestrial equator, 

 where a needle balances itself perfectly 

 horizontally. It is sometimes called the 

 magnetic equator. 



ACORA'CEiE. An order of Mono- 

 cotyledonous plants, named from the 

 genus Acorns. Rhizoma jointed ; leaves 

 ensiform ; flowers hermaphrodite, sur- 

 rounded with scales ; spathe leaf-like ; 

 stamens with 2-celled anthers, turned in- 

 wards ; ovaries distinct ; fruit finally 

 juiceless ; seeds albuminous. 



A'COTYLE'DON (a, priv., kotuX»i5wi/, 

 a seed-lobe). A plant whose embryo 

 has no cotyledons, or seed-lobes. But 

 the acotyledonous embryo is not exactly, 

 as its name seems to indicate, an embryo 



