ACH 



17 



ACI 



These shells are very small, but remark- 

 able for the beauty of their colours ; they 

 are all inhabitants of the Pacific Islands, 

 wher they are used as beadlike orna- 

 ments. 



ACHER'NER, a star of the first magnitude 

 in the southern extremity of the constel- 

 lation Eriilaiius. 



ACHEKSET, an ancient measure of corn, ! 

 supposed to be about eight bushels. 



ACUI'COLU M, the sudatorium of the an- 

 cient baths. 



ACHILLEA. millefoil, yarrow ; a genus 

 of plants of the class gyngeiiesia, and order 

 polyyamia superflim. There are fifty-three 

 species, all, with one exception, hardy , 

 perennials. Only three species are pecu- 

 liar to Britain, sneeze-wort (A. ptar 1 - 

 mica), yarrow (A. mill efo' Hum}, woolly ! 

 millefoil or yarrow (A. tomentosa) in : 

 meadows and pastures, and waysides. I 

 Name from Achilles, who is said to have 

 made his tents of it. 



ACHILLE'IOX, a sort of sponge used by 

 the ancients to make tents for wounds. 



ACHILLES. In anatomy, a strong tendon 

 of the heel is called tendo Achilla. Fable ! 

 says it was by this tendon that Thetis, 

 the mother of Achilles, held him when she ! 

 dipped him in the river Styx, to render | 

 hini invulnerable. 



ACHIOTE, a name of the anuotta tree 

 (bixa orUana\. 



ACHIKITE, " emerald malachite;" a mi- 

 neral consisting of oxide of copper, car- 

 bonate of lime, silica, and water. 



ACHI'KOPOKTOUS, Gr. ^\*ip07rotr)TO?, 

 not made with bauds. Achiropoetous 

 paintings were certain pictures of Christ 

 said to have been painted in a miraculous 

 manner. 



ACH'LYS (**?), darkness. Any 

 opacity of the cornea of the eye. 



ACU'MIT, a mineral of a brownish-black 

 or reddish-brown colour. It is considered 

 a bisilicate of soda. 



ACHNODON'TOS, a genus of plants of the 

 class triundria, order digynia. There are 

 three species, one of which belongs to 

 Britain, the land A. (A. arenarium), a 

 hardy annual. 



ACHOLOVS, Lat acholut, deficient in bile. 



A'cuoB, Gr. 't,\JP- 1. A species of scald- 

 head ; a disease which attacks the hairy 

 scalp of the head, particularly of young 

 children. It is called achor from the 

 brauny scales it throws off. 2. In my- 

 thology, the god of flies, said to have been 

 worshipped by the Cyreneans, to avoid 

 being vexed by them. 



ACH'RAS, a genus of shrubaceous plants 

 Of the class hexandria, order monogynia. 

 In this country they are stove plants. 

 The sapota plum is the fruit of the West 

 Indian species (A. tapota). 



ACHROMA'TIC, from a, viiT.and Xpw/na, 



colour. " Without colour, a term applied 

 to those telescopes formed by such a com- 

 bination of lenses as separates the various 

 coloured rays of li?ht to equal angles of 

 divergency, at different angles of refrac- 

 tion of the near ray, and thereby correct* 

 in a great measure the optical aberration 



' arising from the various colours of light. 



; ACHYRAN'TUES, chaff flower (tt^vpov), 

 chaff, and nvt)o<;, a flower. The descrip- 

 tive name of a genus of hot-house plants. 

 Class pentandria, order monogynia. There 

 are seven species. 



ACIC'CLAR, from acicula, in the shap4 ot 

 a needle. Acicularly, needle-like. 



A'ciD, Lat. acidum (Sax. aced, vinegar). 

 In a popular sense, the word acid is 

 synonymous with *our; but the term is 

 applied in chemistry to several sub- 

 stances which are not so. The general 

 characteristic properties of acids are 

 these, 1. Their taste is generally sour, and 

 in the stronger it is acrid and corrosive. 



2. They unite with water in almost 



any proportion, with a condensation of 



volume and evolution of heat. 3. They 



generally change the vegetable blues to a 

 red. 4. They unite with the alkalies, 

 earths, and metallic oxides in definite 

 proportions, and form salts. This may be 

 reckoned their indispensable property. 



The salts produced by any acids which 

 terminate in ova, have their termina- 

 tion in ite, e. g. the combination of rul- 

 phurout acid and potassa is a sulphite of 

 potassa ; and when an acid whose name 

 terminates in ic, enters into combina- 

 tion, the salt produced has the termina- 

 tion ate, e. g. sulphuric acid and potassa 

 produce sulphate of potassa. Ic gene- 

 rally indicates an acid with much 

 oxygen, e. g. sulphuric acid : out indi- 

 cates a smaller quantity of oxtgen, *. g. 

 sulphurous acid. Hypo prefixed to the 

 name indicates that the acid has a 

 smaller quantity of oxygen than that 

 to which the prefix is made, e. g. hypo- 

 fiilphuric acid, and hyposulphurous acid. 

 Suit is occasionally employed to denote 

 an intermediate degree of oxidation, 

 e. g. tubsulphurout acid, which contains 

 less oxygen than sulphurous, and more 

 than hyposulphunms acid. Per is used 

 where acids have been found to contain 

 more oxygen than those whose name* 

 terminate in ic, e. g. perchloric acid. 

 Hyper where a still larger quantity i 

 observed. Acids are usually divided 

 into two classes, oxacidt and hydracids. 

 The first class includes all those acids, 

 whi.h contain oxygen, and the second 

 tho;-e which contain hydrogen. There 

 are acids, howev r, which belong to> 

 neither of these classes. The term 

 aqueous is now used to designate definite 

 combinations with water: the term 



c 



