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tions themselves cannot wholly remedy 

 the evil ; for " definitions consist of 

 words, and words produce words; so 

 that recourse must be had to particular 

 instances." 



4. Idols of the Theatre ; or the preju- 

 dices and perversions of the mind arising 

 from fabulous and visionary theories and 

 romantic philosophies. They are thus 

 named, "because all the systems of phi- 

 ' losophy which have been hitherto in- 

 vented, or received, are but so many 

 stage-plays which have exhibited nothing 

 but fictitious and theatrical worlds ; and 

 there may still be invented and dressed 

 up numberless other fables of the same 

 kind." 



I'DRIALINE. A substance obtained 

 from a mineral from the quicksilver 

 mines at Idria in Carniola. It consists of 

 carbon and hydrogen. 



IGASURIC ACID. The name given 

 by Pelletier and Caventou to a peculiar 

 acid, which occurs in combination with 

 strychnia in nux vomica, and the St. Ig- 

 natius's bean ; but its existence, as dif- 

 ferent from all other known acids, is 

 doubtful. It is so called from the Malay 

 name by which the natives in India de- 

 signate the faba Sancti Ignatii. 



FGNEOUS ROCKS {ignis, fire). A 

 term applied to all rocks, as lava, trap, 

 and granite, which are known or sup- 

 posed to have been melted by volcanic 

 heat. 



IGNIS FATUUS. A luminous ap- 

 pearance or flame, frequently seen in 

 the night in the country, and called Jack 

 o' lantern, or Will with the wisp. It is 

 probably occasioned by the extrication 

 of phosphorus from rotting leaves and 

 other vegetable matters. 



IGNITION {ignis, fire). The emission 

 of light from a heated body, unattended 

 by change of composition. Bodies begin 

 to become ignited, or red-hot, at about 

 800° Fahr. ; the highest point of ignition 

 consists in the emission of a perfectly 

 white light. 



I'GREUSINE. Elaiodon. That por- 

 tion of volatile oils which is odoriferous, 

 and is coloured by treating it with nitric 

 acid. 



IGUA'NIDiE. A family of saurian 

 reptiles, including some of the largest of 

 this tribe, both recent and fossil. They 

 agree with the lizards in general form, 

 but diflfer in the shortness and thickness 

 of their tongues. See Iguanodon. 



IGUA'NODON {iguana, a recent West 

 Indian lizard, o6ovi, a tooth). A gigantic 

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fossil saurian, exceeding eighty feet in 

 length, occurring in the Wealden forma- 

 tion, and known to us by the teeth and a 

 considerable part of the skeleton. Its 

 name is derived from the resemblance of 

 its teeth to those of the iguana. 



I'LLATIVE CONVERSION. In Logic, 

 a mode of conversion in which the truth 

 of the converse follows from the truth of 

 the exposita, or proposition given. Con- 

 version can then only be illative, when 

 no term is distributed in the converse, 

 which was not distributed in the expo- 

 sita. Illative conversion is not a process 

 of reasoning ; it is only stating the same 

 judgment in another form. See Con- 

 version. 



ILLUMINA'TION {illumino, to throw 

 light upon). The process of rendering a 

 body visible by light: every object ex- 

 posed to the sun is illuminated ; a lamp 

 illuminates a room and every object 

 in it. 



IMAGE. The appearance of an ob- 

 ject made either by reflection or refrac- 

 tion. In all plane mirrors, the image is 

 of the same magnitude as the object, and 

 it appears as far behind the mirror as the 

 object is before it. In concave mirrors 

 the image appears larger, and in those 

 which are convex it appears less, than 

 the object. 



Image, Aerial. When an object is 

 placed at a greater than its focal distance 

 from a convex lens, it produces on the 

 opposite side of the glass an inverted 

 image floating in the air; hence called 

 an aerial image. 



IMA'GINARY QUANTITIES. In 

 Algebra, this term is applied to the even 

 roots of negative quantities, as s/ — x^. 

 Such expressions indicate operations 

 which are impossible, and hence they are 

 also called impossible quantities. The 

 difference between surd and impossible 

 quantities is, that the former have real 

 values, though we cannot exactly find 

 them, while there cannot be a quantity, 

 positive or negative, an even power of 

 which would produce a negative quan- 

 tity. 



IMA'GO. A term applied to the third 

 and perfect state of insect existence, in 

 which, the skin of the pupa having 

 burst, the animal escapes, furnished 

 with wings adapted for flight. 



IMBIBI'TION {imbibo, to drink in). 

 The terms imbibition, and exudation or 

 transpiration, used in Physiology, are 

 analogous to those of aspiration and ex- 

 piration, and have been lately translated, 

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