TNU 



lOD 



rived ; its ratio is that of 1 ; 2 I 4 ; 8, 

 &c. Compound intervals are those which 

 exceed an octave, and they are named 

 according to the distance of the two 

 boundary notes. 



INTERVE'NIUM {inter, between, 

 vena, a vein). That portion of the par- 

 enchyma of leaves, which lies between 

 two or more veins or veinlets. 



INTESTI'NA. An order of worms 

 which inhabit the bodies of other ani- 

 mals. They are distinguished by Cuvier 

 into the Cavitaria, which have cavities 

 or stomachs ; and the Parenchymata, or 

 cellular-bodied, as the tape-worm. 



INTE'XINE. That coating of the 

 pollen-grain which is situated next to 

 the extine, constituting a fourth layer of 

 the pollen-grain in certain plants. 



I'NTINE. The inner coat of the shell 

 of the pollen-grain in plants. See Ex- 

 tine. 



I'NTROIT {ivtroitus, an entering in). 

 A term employed in ancient chronicles, 

 signifying the first two or more words 

 which form the commencement of a 

 mass, which, from being appropriated to 

 a certain Sunday, or other festival, gave 

 the name of such commencement or 

 " introit " to those days. Thus, the term 

 " adorate Dominum" is the introit and 

 name of the third Sunday after the Epi- 

 phany. 



INTRO'RSE {introrsus, qu. intro- 

 versus, inwardly). Turned inwards; as 

 the anthers of plants in which the line of 

 dehiscence is towards the axis of the 

 flower. See Extrorse. 



INTUITION {intueor, to look into). 

 That simple faculty of the mind by 

 which we immediately perceive the 

 agreement or disagreement of two ideas. 

 In this the mind is at no pains of proving 

 or examining, but perceives the truth as 

 the eye does the light, only by being 

 directed to it. Intuitive truth has ac- 

 cordingly been defined as that " which is 

 perceived immediately on a bare atten- 

 tion to the ideas under review." 



INTUS-SUSCEPTION {intus, within, 

 suscipio, to receive). The inversion of a 

 part of a tube within a contiguous part. 

 When it takes place downwards, it may 

 be termed progressive ; when upwards, 

 retrograde. The term intussusception is 

 also applied to the process of nutrition, 

 or the transformation of the components 

 of the blood into the organized substance 

 of the various organs. 



I'NULIN. A starch-like substance 

 which is spontaneously deposited from a 

 183 



decoction of the root of the Inula Hele- 

 nium, or Elecampane. 



INVAGINATION {in, in, vagi nd, a 

 sheath). A sheathing of one part within 

 another ; a term synonymous with intus- 

 susception. 



INVA'RIABLE. In Mathematics, a 

 terra synonymous with constant, and 

 applied to a quantity which is absolutely 

 invariable. It may also denote a func- 

 tion which is not absolutely invariable, 

 but which does not vary in the processes 

 required by a given equation. 



INVE'NTION. A term frequently 

 confounded with discovery, which simply 

 means the finding out something already 

 existing. Invention, however, includes 

 conception, and relates to the mental 

 operation by which obj cts are selected 

 and arranged with reference to a parti- 

 cular result. The polarity of the magnet 

 was discovered ; its application to navi- 

 gation was invented : the binomial theory 

 was a discovery ; the method of fluxions 

 an invention. 



INVERTEBRATA. A negative and 

 unsatisfactory term by which Lamarck 

 designated all those animals which are 

 not furnished with a vertebral column 

 or backbone. 



INVOLU'CRUM {involvo, to wrap in). 

 In Botany, a whorl of bracts which sur- 

 round several flowers, as in composite 

 and umbelliferous plants. In the latter 

 family, the bracts which surround the 

 general umbel are called the universal 

 involucrum ; and those which surround 

 the umbellules, a partial involucrum, or 

 involucellum. 



I'NVOLUTE. That kind of curve 

 which is described by the extremity of a 

 cord as it is unrolled from the arc of 

 another curve about which it has been 

 lapped, the latter being called the evolute. 

 The two may be described as, the curve 

 unrolled, and the curve from which it is 

 unrolled. See Curve. 



INVOLUTION (in Algebra). The 

 process of finding the powers of quan- 

 tities. The examples of involution are, 

 therefore, only examples of multiplica- 

 tion where the factors are all the same. 

 See Evolution. 



rODINE {\u)8t]f, or ioet3>j?, violet- 

 coloured, from 'iov, a violet, and eJdo?, 

 likeness). A non-metallic, crystallized, 

 solid substance, found in marine plants, 

 in the ocean, and in mineral springs ; it 

 becomes volatile by a slight increase of 

 temperature, and forms a beautiful violet 

 vapour. 



