I MP 



INC 



by Dutrochet, by the two Greek words 

 endosmosis and exosmosis. 



I'MBRICATED {imbrex, a roof-tile). 

 A designation of that form of aestivation, 

 or vernation, in which the pieces of the 

 bud overlap each other parallelly at the 

 margins, without any involution. The 

 term is particularly applicable to the 

 bracts of gluraaceous plants. 



IMMER'SION {immergo, to plunge 

 into). An astronomical term, denoting 

 the disappearance of any celestial body 

 behind another or in its shadow, as in 

 the eclipses of Jupiter's satellites. The 

 re-appearance of the body, after occulta- 

 tion, is termed its emersion. 



I'MPACT {impingo, to impinge). By 

 Impact or Percussion is meant the colli- 

 sion of two bodies, of which one at least 

 is in motion. The impact is said to be 

 central, when the directions of the 

 centres of gravity of the two solid bodies 

 are in a right line ; eccentric, when this 

 is not the case ; direct or perpendicular, 

 when the direction of the moving body is 

 at right angles to the side of the body 

 impinged on ; oblique, when this is not 

 the case. 



I'MPARI-PINNATE. A term ap- 

 plied, in Botany, to that mode of the 

 composition of leaves, in which they are 

 pinnate with an odd one, as when the 

 petiole of a pinnate leaf is terminated by 

 a single leaflet, as in mountain-ash. 



IMPE'NETRABl'LITY {in, not, pene- 

 troy to penetrate). That property by 

 which a body occupies any space, to the 

 exclusion of every other body. In a 

 popular sense, all matter is penetrable; 

 but, philosophically speaking, it is im- 

 penetrable, what is called penetration 

 being merely the admission of one sub- 

 stance into the pores of another. A nail 

 driven into a board or piece of lead, does 

 not penetrate the wood or metal; it 

 merely separates or displaces the particles 

 of these substances. 



IMPE'RATIVE MOOD {impero, to 

 command). That mood of the verb 

 which commands an action to be per- 

 formed, or a state to exist. 



IMPO'NDERABLES (in, priv., pon- 

 dus, weight). Agents which are desti- 

 tute of weight, as heat, light, and elec- 

 tricity. These are supposed to be of so 

 subtile a nature as to pass through all 

 material bodies, by which, therefore, they 

 cannot be either enclosed or divided. 



IMPOSSIBILITY. This term is used 

 in three different senses : — 



1. A mathematical impossibility is that 

 176 



which involves an absurdity and self- 

 contradiction, as that two straight lines 

 should enclose a space. No limitation 

 of power is here implied ; it is, in reality, 

 nothing that is required to be done. 



2. A physical impossibility is some- 

 thing at variance with the existing laws 

 of nature, and which, consequently, no 

 being, subject to those laws, can sur- 

 mount; but we can easily conceive a 

 Being capable of bringing about what in ' 

 the ordinary course of nature is impos- 

 sible. An occurrence of this character 

 we call miraculous. 



3. A moral impossibility is that high 

 degree of improbability which leaves no 

 room for doubt. This implies no contra- 

 diction, nor any violation of the laws of 

 nature, but which yet we are rationally 

 convinced will never occur, merely from 

 the multitude of chances against it ; as 

 that unloaded dice should turn up the 

 same faces one hundred times succes- 

 sively. And yet why should they not ? 

 since the chances are the very same 

 against any given one hundred throws. — 

 Whately. 



IMPOSSIBLE. In Logic, the matter 

 of a proposition is said to be impossible, 

 when the extremes altogether disagree. 

 The proposition is then equivalent to a 

 universal. Thus, brutes neither eat nor 

 drink more than nature requires ; that is, 

 no brutes, 8fc. 



An impossible quantity, in Algebra, is 

 the same as an imaginary quantity. See 

 the latter term. 



I'MPULSE {impulsus, a push or stroke). 

 The direct action of one body upon an- 

 other in the production cf motion. Bodies 

 are impelled, or driven forward, either 

 by percussion or by pressure — by a stroke, 

 as by a hammer, or by a push, as by a 

 spring or living power. The former is 

 instantaneous, the latter continuous. In 

 both cases the moving body flies from 

 the power; in the action of pulling, or 

 attraction, it does the reverse. 



INCANDE'SCENCE {incandesce, to 

 become white-hot). The glowing or 

 shining appearance of heated bodies; 

 properly, the acquisition of a white 

 heat. 



INCEPTIVE {incipio, to begin). That 

 which begins any thing ; a term some- 

 times applied to a moment or principle, 

 from which, as from a starting-point, 

 something may be extended; thus a 

 point or a line, in mathematics, though 

 possessing no magnitude, is said to be 

 inceptive of enlargement. But this is a 



