IMP 



IMP 



which so fills a space as to exclude all 

 others, is said to be impenetrable. Or, 

 by impenetrability is meant, the .'faculty 

 which a body has of excluding every 

 other body from the place that it occu- 

 pies, in such manner that two bodies 

 placed in contact can never occupy less 

 space than that which they filled when 

 they were separate. The impenetrability 

 of solid bodies does not require to be 

 proved, it strikes us at first view ; but 

 fluids, having their particles perfectly 

 moveable in every direction, and yielding 

 to the slighest pressure, their impenetra- 

 bility does not manifest itself so percep- 

 tibly as that of solid bodies. Taking the 

 air for an example : so long as this fluid 

 is not enclosed in something, its extreme 

 mobility causes it to admit a free passage 

 to all bodies which are moved through it; 

 but in this case it is properly displaced, 

 and not penetrated; for, if the air be in- 

 cluded within the sides of a vessel, and 

 <f another body be then presented to take 

 its place, without suffering it to escape, 

 it will exercise its impenetrability in the 

 same manner as solid bodies. It is easy 

 to be convinced of this by the aid of a 

 very simple experiment, which any one 

 may make : it consists in plunging a ves- 

 sel vertically, with the orifice downwards, 

 in another vessel filled with water to a 

 certain height : the surface of the water, 

 corresponding with the orifice of the first 

 vessel, is depressed as this vessel itself 

 descends; and this depression may be 

 rendered more sensible by means of a lit- 

 tle plate, or slip of cork, placed so as to 

 float upon the surface of the water ; 

 nevertheless, this water is not excluded 

 by the air occupying the immersed ves- 

 sel ; it is always raised within it by a cer- 

 tain quantity, which augments as the ves- 

 sel is immersed to a greater depth : but 

 it is sufficiently evident that this ascen- 

 sion is occasioned by the circumstance 

 that the air is a compressible fluid, and 

 therefore its volume is contracted into a 

 smaller space, by the effect of the com- 

 pression excited upon it by the surround- 

 ing water on all parts, in virtue of its 

 weight. We must here notice a difficulty 

 Which appears to result from this, that 

 when we have mingled certain bodies, 

 the volume of the mixture is less than the 

 sum of the volumes taken separately. 

 This happens, for example, when we mix 

 equal parts of alcohol and water ; the 

 same also obtains, when we mingle, by 

 fusion, copper with zinc, in order to form 

 the compound metal called brass : it is 

 then observed, that the density of the 



mixture is augmented by about its tenth 

 part. This apparent penetration is ow- 

 ing to the circumstance, that the molecu- 

 Ix of the two bodies, in consequence of 

 their respective formation, generally ap- 

 proach one another more than in the two 

 bodies taken separately ; there hence re- 

 sults, in the figure of the pores, such a 

 change as diminishes the space equal to 

 the sum of these pores. On the contrary, 

 in the alloy of silver with copper, a kind 

 of rarefaction is produced, such that the 

 volume of the mixture is larger than the 

 sum of the volumes of the two bodies, 

 previous to fusion. 



IMPERATIVE, one of the moods of a 

 verb, used when we would command, en- 

 treat, or advise : thus, 5-0, ready take pity, 

 be advised, are imperatives in our lan- 

 guage. 



IMPERATORIA, in botany, a genus of 

 the Pentandria Digynia class and order. 

 Natural order of Umbellatse, or Umbelli- 

 ferae. Essential character: fruit round- 

 ish, compressed, gibbose in the middle, 

 surrounded by a margin ; petals inflex, 

 emarginate. There is but one species, 

 viz. I. ostruthium, master-wort, the root 

 of which is thick, running obliquely in 

 the ground ; it is fleshy, aromatic, and has 

 a strong acrid taste, biting the tongue and 

 mouth ; the leaves arise immediately 

 from the root, having foot-stalks seven 

 inches long, divided into three very short 

 ones at the top, each sustaining a trilo- 

 bate leaf, indented on the border; the 

 foot-stalks are deeply channeled, and 

 when broken emit a rank odour;the flow- 

 er stalks rise two feet high, dividing into 

 two or three branches, each terminated 

 by a pretty large umbel of white flowers, 

 whose petals are split ; these are suc- 

 ceeded by oval compressed seeds, resem- 

 bling those of dill, but larger. Linnaeus 

 observes, that the floral leaves are oppo- 

 site, that there is a petiolary, membrana- 

 ceous, ventricose, stipule, one within an- 

 other: It is a native of many parts of the 

 Alps, Austria, Syria, Tyrol, Silesia, and 

 Dauphine. 



IMPERFECT, something that is defec- 

 tive, or that wants some of the properties 

 found in other beings of the same kind : 

 thus mosses are called imperfect plants, 

 because almost all the parts of fructifica- 

 tion are wanting in them ; and for the like 

 reason is the appellation imperfect given 

 to the fungi and submarine plants. 



IMPERFECT numbers, such whose aliquot 

 parts taken together'do either exceed, or 

 fall short of that whole number of which 

 they are parts : they are either abundant 



