12 



EXPLANATION OF SCIENTIFIC TERMS. 



ICELAND SPAR. See Calcareous Spar, 

 and Spar. 



ICOSAHEDRON. See Rhombus. 



IDOCRASE, "a name sometimes given to 

 Vesuvian, which see. 



IMPENETRABILITY. In the popular 

 acceptation every substance is penetrable, 

 that is, another substance may be intro- 

 duced, or made to pass through it, pro- 

 vided a sufficient force be applied ; but 

 the presently received system of philoso- 

 phy holds all matter to be impenetrable, 

 and that what is supposed to be penetra- 

 tion is merely the admission of one sub- 

 stance into the pores of another. 



IMPULSE (Latin impulses, a push or 

 stroke) is the direct action of one body 

 upon another in the production of motion. 

 Bodies are impelled, or driven forward, 

 either by percussion, or by pressure, by 

 a stroke, as with a hammer, or a push, as 

 by a spring, or a living power. The 

 former is instantaneous, and the latter 

 continuous. In both cases the moving 

 body flies from the power ; in the action 

 of pulling (or attraction) it does the re- 

 verse. 



INCIDENCE, POINT OF. See Refrac- 

 tive Power. 



INCIDENCE, ANGLE OF. See Re- 

 flexion, and Refractive Power. 



INCOMMENSURABLE. See Ratio. 



INCOMPRESSIBILITY. See Compres- 

 sibility. 



INSTRUMENT. See Machine. 



INDEX OF REFRACTION. See Re- 

 fractive Power. 



INERTIA. See Vis Inertia. 



INSULATION. When a body, contain- 

 ing a quantity of free caloric, or of the 

 electric fluid, is surrounded by non-con- 

 ductors, so as to cut off the communica- 

 tion with other bodies, it is said to be 

 insulated; a metaphor taken from the 

 Latin insula, an island. 



INVERSE PROPORTION, or RATIO. 

 See Ratio. 



IN VOLUTE OF A CURVE. See Curves. 



ISOCHROMATIC. The Greek i*os sig- 

 nifies equal, and is prefixed to many 

 scientific words which are derived from 

 that language. Isochr&matic is, having 

 the same colour. Isoperimetrical is, 

 having the same length of perimeter, or 

 bounding line. Isochronous is what 

 passes in equal times, as the vibrations of 

 pendulums of the same length, &c. 

 JARGON. See Zircone. 



INT HEAT. See Caloric. 

 T OF THE SINES. See Refractive 

 )er. 



(Latin lens, a bean) is properly a 

 11 roundish glass of the figure of a 

 /. The meaning, however, is now 

 nded. Lenses are not now necessa- 

 glass, nor shaped like a bean, but 

 be made of other forms, and of any 

 iparent substance. Their essential 



characteristic is, that they shall refract 

 the rays of light, so that the divergence, 

 or convergence, of those rays shall be 

 equally produced after their passage. For 

 this purpose the surfaces must be po- 

 lished. Their usual forms and names 

 are shown and explained at page 1, 

 OPTICS. 



LEVEL. Two points on the surface of 

 the earth ai'e said to be on the same level 

 when they are equally distant from its 

 centre. A level surface, therefore, is 

 not a plane, but a portion of a spherical 

 surface ; and this is the form which a 

 sheet of water, or any other liquid, natu- 

 rally assumes. There are various instru- 

 ments used in levelling, which are called 

 levels. These all give a horizontal level, 

 that is, a tangent to the earth's surface ; 

 and in the case of a drain, or canal, the 

 bottom of the excavation must be carried 

 lower than the level indicates, otherwise 

 the water would not run. The declivity 

 must be in a circle equivalent to that of 

 the earth's circumference before the water 

 could reach it, and this would then be 

 termed a dead level. 



LEUCOCYOLITE, a name given to 

 variety of Apophyllite, which see. 



LEVER ( Latin levare, and French lever, 

 to lift, or raise) is one of the mechanical 

 powers. It is an inflexible bar, supported 

 and moveable in one point of its length 

 on a pivot, or prop, called the Fulcrum. 

 One end of the lever is applied to the 

 weight to be raised, while a force is ap- 

 plied to the other end. The power of this 

 instrument depends on the proportion 

 between the lengths of the parts of the 

 lever on each side of the fulcrum. See 

 Balance. 



LIGHT is the cause of those sensations 

 and colours which we refer to the eyes, 

 or sense of seeing, as their source. The 

 essence of light is unknown : whether it 

 consists of emanations from the substance 

 of the luminous body, or is propagated, 

 by impulse, through the medium of an 

 universally diffused and subtile etber, has 

 not yet been determined. The knowledge 

 of the laws which regulate the phenomena 

 of light constitutes the science of Optics , 

 the investigation of its action upon the 

 structure of bodies belongs to Chemistry. 



PENCIL OF. See Light, Ray of. 



RAY OF, is considered as an 



evanescent element of a stream of light ; 

 and a pencil as a collection of such rays 

 accompanying each other. 



REFLEXION AND REFRAC- 

 TION OF. See Reflexion and Refrac- 

 tion. 



LIMIT. A limit is literally a boundary, 

 from the Latin limitare, to bound. There 

 are certain effects in Natural Philosophy, 

 as well as quantities in Mathematics, 

 which we cannot determine with minute 

 accuracy ; but, in many such cases, we 



