IDE 



I DO 



indistinct ; the maxillary and the inter- 

 maxillary bones are either wanting or 

 rudimentary, their place being supplied 

 by the palatine or vomer. The orders are 

 the Eleutheropomi, Plagiostomi, and Cy- 

 clostomi. 



ICHTHYOPHTHA'LMITE (Ix6'up, a 

 fish, 6(p0aXiJLos, an eye). Fish-eye stone ; 

 a species of zeolite, with a pearly lustre, 

 resembling the eye of a fish. See Apo- 

 phyllite. 



ICHTHYOSAU'RUS {'ixOv^, a fish, 

 aav(>oi, a lizard). A gigantic fossil ma- 

 rine reptile, intermediate in its characters 

 between a crocodile and a fish, occurring 

 in the lias formation. It differs from 

 any known reptile of the present day, in 

 the substitution of paddles for feet. 



I'COSAHE'DRON {eUoci, twenty, 

 '4bpa, a seat). In Geometry, a solid figure 

 bounded by twenty equal and equilateral 

 triangles. 



ICOSA'NDRIA (eiKOfft, twenty, avijp, 

 a man). The twelfth class of plants in 

 the system of Linnaeus, characterized by 

 the presence of twenty or more stamens 

 inserted into the calyx. 



IDEA (Idea, form). In its widest accep- 

 tation, this term denotes an impression 

 of an external object upon the senses, or 

 of an object of thought upon the mind. 

 In the Platonic philosophy the \beat, or 

 *' Ideas," were not only e'ibn, or species, 

 but something more, viz. the perfect 

 archetypes, models or patterns, of which, 

 respectively, all created things were the 

 imperfect anti-types or representations; 

 ei5op might therefore be used for idea, 

 but not Idea for €i5of. 



IDE'AL THEORY. A scholastic 

 theory, which refers all perception to the 

 impression on the mind of certain ideas 

 or images of external objects, the organs 

 of sense having no percipient power them- 

 selves, but merely transmitting the ideas 

 of these objects to the mind. 



IDE'NTICAL {idem, the same). 1. In 

 its primary and proper sense, this term 

 denotes oneness or sameness, and is ap- 

 plicable only to a single object. 2. In its 

 secondary and improper sense, it is em- 

 ployed to denote great similarity, as 

 "When two guineas, struck from a wedge 

 of uniform fineness, are said to be " of one 

 and the same form and weight, and also of 

 one and the same substance." Yet they 

 are not identical, but numerically dis- 

 tinct. Further, in this secondary sense, 

 the term popularly admits of degrees : 

 we speak of two things being nearly iden- 

 tical, nearly the same, but not entirely ; 



whereas personal identity does not admit 

 of degrees.— Whately. 



IDEOGRA'PHIC WRITING {Idea, 

 an idea, ypa(p(a, to write). A kind of 

 writing by which objects or abstract ideas 

 are symbolically represented, as in the 

 figurative part of the Egyptian hierogly- 

 phics. It is opposed to phonetic writing, 

 in which the symbols employed represent 

 sounds. 



IDEO'LOGY {Idea, an idea, \6yo9, a 

 description). By this term the later dis- 

 ciples of Condillac have designated the 

 science of ideas, or the history and evo- 

 lution of human ideas, considered as 

 successive modes of certain original or 

 transformed sensations. 



IDIOSY'NCRASY ('idio<f, peculiar, o-i^y- 

 Kpaaii, composition). Individual pecu- 

 liarities, hereditary or induced. Thus, 

 there are persons in whom opium does 

 not induce sleep ; others, in whom milk 

 seems to act as a poison. 



I'DOCRASE (et3or, form, Kpacru, mix- 

 ture). A designation of Vesuvian or 

 Pyramidal Garnet, from the mixed forms 

 of other minerals which it presents. It 

 is an alumino-silicate of lime, with a 

 portion of oxide of iron. 



I'DOLS {e'l6(o\ov, an image). By this 

 fanciful term Lord Bacon points out the 

 various general sources of those errors 

 which we are apt to commit in forming 

 our notions of things. These idols, or 

 false notions of the mind, are divided 

 into four classes, viz. : — 



1. Idols of the Tribe ; so called because 

 they are common to the whole tribe or 

 race of mankind, being, in fact, those 

 general prejudices which arise from the 

 infirmity of human nature itself. " The 

 understanding of man is like a mirror 

 whose surface is not true, and so, mixing 

 its own imperfection with the nature of 

 things, distorts and perverts them." 



2. Idols of the Cave or Den ; or "each 

 man's particular demon, or seducing 

 familiar spirit;" or, less figuratively, 

 those prejudices which stamp upon each 

 mind its own peculiar character, and are 

 identified with every individual man. 

 Every mind is compared to " a glass, 

 with its surface differently cut, so as dif- 

 ferently to receive, reflect, and refract, 

 the rays of light which fall upon it." 



3. Idols of the Market-place; or pre- 

 judices arising from mere words and 

 terms in our common intercourse with 

 mankind ; these proceed, in short, from 

 the imperfection of language, and are 

 " the most troublesome of all." Defini- 



