IMPACT. 



IMPULSE. 



830 



IMPACT. [COLLISION, IMPACT, or PERCUSSION OF BODIES.] 



IMPEACHMENT. [PARLIAMENT.] 



IMPENETRABILITY, a name given to the property of matter, the 

 existence of which is suggested when we see that any attempt to place 

 one solid body in the part of space occupied by another is either 

 resisted by the latter, or its success preceded by the removal of the 

 latter. It is then but another name for the cause of that resistance, 

 which we know by the sense of touch, and which is necessary to every 

 idea which we form of matter. 



The impenetrability of matter can only be taken in conjunction with 

 the hypothesis of its porosity. Otherwise, it might be successfully 

 disputed. Salt may be dissolved in water without increasing the bulk 

 of the fluid ; the (impenetrable ?) matter in then penetrated ; or else 

 the matter of the fluid has interstices. But if any attempt be made to 

 press the fluid into a smaller space, the impenetrability of the water 

 will appear by its resistance to the pressing substance. 



Are we not then making a purely gratuitous introduction of words 

 to supply explanations of phenomena ? When matter resists, we have 

 recourse to impenetrability, which is merely saying, so soon as we find 

 resistance, that matter has a power of resisting. But when we ascertain 

 that different portions of matter can be made to fill the same fpice, 

 whether by solution, pressure, or otherwise, we then appeal to a 

 porosity which we cannot make visible, and presume that matter has 

 empty spaces in which other matter may be placed. This is very 

 much like nature's horror of a vacuum, and other explanations of the 

 same kind. 



The answer to this difficulty, and others of a similar kind which 

 occur in attempting to define simple mechanical terms, is that the 

 beginner must not receive them as explanations or as doctrines, but 

 simply as statements of observed phenomena, or at best as terms which 

 imply that explanation is wanted, and serve, till further explanation, 

 to enable us to recal the phenomena themselves and the universality 

 of their existence. Subsequent study and experiment must ascertain 

 the character of this impenetrability, that is to say, the laws of the 

 resistance from which it derives its name. The term is useful to 

 remind us that there is a something which shows itself in all matter ; 

 but neither its etymology nor any common notions attached to it must 

 be allowed to dictate any conclusion as to the nature, mode of action, 

 or consequences of that something. 



A closer inquiry into the action of matter upon matter shows us that 

 the fundamental notion upon which the above-mentioned something 

 received the name of impenetrability is incorrect. That notion 

 evidently was, that when, for instance, one ball is rolled towards 

 another, the rolling ball absolutely touches the stationary one before it 

 causes motion. There are many circumstances from which it can be 

 inferred, with the highest degree, of probability, that such contact is only 

 apparent, and not real. It must be concluded that when the two balls 

 come within a certain small distance of one another, repulsive forces, 

 of the cause and mode of action of which we know nothing, begin to 

 be excited between those particles of the balls which are nearly in 

 contact. It is not our intention in this article to go further into the 

 preceding subject than the mere mention of what are called mokcular 

 attractions and repulsions, and this merely to point out that the action 

 of .matter upon matter must be admitted to commence before the 

 instant at which their surfaces come into contact. [ATTRACTION; 

 IHERTIA.] 



IMPERATOR. [EMPEROR.] 



IMPERATORIN, Pcucedanin, (C^H^O,), a neutral crystalline vege- 

 table product, procured from the root of the Imperatoria Oitruthium : 

 it is obtained by exhausting the root with ether, distilling the greater 

 part of it, and submitting the remainder to spontaneous evaporation, 

 by which the imperatorin is obtained crystallised in prisms, which are 

 colourless, transparent, inodorous, styptic, and acrid ; they do not alter 

 vegetable colours; they melt when heated to 167, and concrete into 

 a radiated mass, the density of which is 1-192. At a high temperature 

 imperatorin is decomposed, diffusing an acrid vapour. It is insoluble 

 in water, but soluble in alcohol, ether, olive oil, and oil of turpentine ; 

 when dissolved in a solution of potash, it ii precipitated from it by 

 acids, unchanged. Sulphuric acid dissolves imperatorin, and assumes 

 a brownish red colour ; in nitric acid it dissolves with a yellow tint : 

 these solutions are both decomposed by water. Iodine forms with it 

 a brownish red compound, which yields iodine when heated. 



IMPERMEABILITY, that property by which certain substances 

 resist the passage through their mass of other substances : such, for 

 example, as glass, the pores of which are so small that under no 

 prewure hitherto applied have fluids been driven through them ; 

 whereas gold is permeable by water, as in the celebrated experiment of 

 the Florentine Academicians. Some substances are impermeable in 

 consequence of their repulsions to other bodies, such as oilskin or 

 waterproof cloth, which is impermeable to water, in consequence of 

 the repulsion existing between oil and water. 



IMPKTl'UO is a terra which has been employed by writers in many 

 independent significations, and for various diseases, but is now confined 

 to a disease of the skin, which Dr. Willan has defined to be " an 

 eruption of yellow itching pustules, appearing in clusters and termina- 

 ting in a yellow, thin, scaly cruet." It is commonly known in this 

 country as the humid or riming tetter, and occurs on all parts of the 

 body, though most frequently on the extremities. 



A variety of it is not unfrequently met with hi grocers and those 

 much engaged in handling sugar, and to this the term grocer's itch has 

 been applied ; but it differs from the itch, properly so called, in its 

 non-contagiousness. [SKIN, DISEASES OF.] 



IMPETUS. [MOMENTUM.] 



IMPONDERABLE, a term applied to heat, light, and electricity, 

 regarded as fluids ; but as in modern science they are regarded rather 

 as forces, the term has gone out of use. 



IMPORTS AND EXPORTS. [TRADE AND SHIPPING.] 



IMPOSSIBLE. [NEGATIVE AND IMPOSSIBLE QUANTITIES.] 



IMPOST (Imposta, Italian), the horizontal mouldings which serve as 

 a sort of cap or cornice to the piers of arches, and on which the archi- 

 volts, or curved mouldings and facia; surrounding the arches them- 

 selves, rest. Like these latter the impost is made plainer or richer 

 according to the order employed, or to the general character of the 

 design. And when the archivolts of the arches are omitted, either tho 

 impost is omitted likewise, or a plain band is substituted for it. This 

 is generally done in the rusticated basements beneath an order, the 

 joints of the rustics sufficing for decoration, and giving the requisite 

 architectural expression. Imposts, properly so called, are seldom found 

 in pointed Gothic, except in the First Pointed style ; but the point of 

 junction between the curve of an arch and its support, however treated, 

 is called the impost. Except in the case above alluded to, the impost 

 is essential in Roman and Greco-Roman architecture. We have, how- 

 ever, a few recent instances in which imposts have been omittud. and 

 the archivolt of the arch continued vertically down the edges of the 

 piers. This was a favourite practice with Soane, both in his designs 

 and many of his executed buildings ; and it has been followed by other 

 architects, but the effect is by no means happy. 



IMI'RKSSMENT. [SEAMEN.] 



IMPRISONMENT, FALSE. [FALSE IMPRISONMENT.] 



IMPROPRIATIONS. [BENEFICE ; TITHES.] 



IMPROVEMENTS, PUBLIC. [PUBLIC IMPROVEMENTS.] 



1MPROVVISATORI are extempore versifiers who can, without 

 preparation, pronounce a certain quantity of verses upon any given 

 subject. This practice is of frequent occurrence in Italy, and the 

 facilities which the structure of the Italian language affords to versi- 

 fication and rhyme are of great assistance towards it. The improvvisa- 

 tore delivers his verse, generally accompanied by a guitar, and with a 

 sort of chaunting cadence ; and he spins .out hundreds, nay at times 

 thousands of lines, with apparent ease : whole dramas have indeed been 

 thus delivered. It must not be imagined however that this kind of 

 extempore poetry is of the best kind ; in reality very few of those com- 

 positions can stand the test of publication. Still they have the merit 

 of the flow of language and the quick adaptation of accessory ideas and 

 images to the main subject, which rivet the attention and excite the 

 surprise of the listener. 



Some improwisatori have been men of real information and poetical 

 genius, and their compositions are consequently superior. An Augustine 

 friar of the name of Brother Philip, blind and living in the time of 

 Sixtus V., is mentioned as having done wonders in this way. Gianni, 

 of Genoa, a man of considerable poetical talent, was made improvvisa- 

 tore to the court of the Emperor Napoleon I., with a handsome salary; 

 and Sgricci of Florence became known throughout Europe by giving 

 specimens of his art in the various capitals. Several ladies have dis- 

 tinguished themselves in the same art : they are styled improwisatrici. 



IMPULSE. When a body rolls down a gently-inclined plane we 

 can see the gradual alterations of its velocity, and can readily admit 

 that between the instants at which the body has two different velo- 

 cities it takes in succession all intermediate velocities, or that the 

 change of velocity is perfectly gradual. But when a body is violently 

 struck, as in the case of a bat and a ball, we can see no gradations of 

 velocity, but the ball appears to be at once altered from a state of rest 

 into one of rapid motion, without having passed through any of the 

 intermediate states. In this case it is said to have received an impulse, 

 which word must be interpreted to mean any cause by virtue of which 

 velocity is communicated suddenly and without gradations. 



Though the term impulse may be of convenient application to cases 

 of motion in which velocities are changed very rapidly, it must bo 

 remembered that the idea of absolutely instantaneous change of velo- 

 city is in no degree less absurd than that of a point which is in two 

 different positions at the same instant of time. Impulse, then, must 

 be considered as pressure which, beginning from nothing, increases so 

 rapidly with the time as to produce large effects in a small fraction of 

 a second. Some account of the manner in which impulses act is given 

 in the article PRESSURE. 



It is to be remembered that there is nothing absurd in the idea of 

 any change in the state of a body, provided that a time, no matter 

 how small, be allowed for it to take place in. A cannon-ball now at 

 rest may, in the millionth part of a second, be imagined to have 

 acquired a velocity such as it has when it issues from the mouth of the 

 gun, provided only that a pressure be imagined sufficient to produce 

 the effect. It is only the production of velocity in no time at all which 

 must not be admitted ; though it must be owned that the excessive 

 smallness of the times in which some pressures produce a great effect 

 makes us familiar with the notion of impulse, which further inquiry 

 shown us to be a mechanical impossibility aa long as the present laws 

 of nature last. 



