10 



EXPLANATION OF SCIENTIFIC TERMS. 



in very thin plates, through quartz, feld- 

 spar, and other fossils ; but it is also 

 found in the form of prismatic crys- 

 tals. It constitutes a principal ingredient 

 in many of what are termed primitive 

 rocks. The plates are sometimes large 

 (as much as eighteen inches diameter), 



\ and extremely thin. They are employed 

 in Russia for window- panes ; and, in 

 that state, are called Muscovy Glass. 



MILLIMETRE. See Metre. 



MINERAL ALKALI. See Alkali. 



MINIMUM. See Maximum. 



MIRRORS, PLANE, CONCAVE, and 

 CONVEX. See Gloss. I. 



MOCHO STONE. See Agate. 



MOLECULE is a diminutive, formed 

 from the Latin motes, a mass, and denotes 

 one of the minute particles of which the 

 mass or hody is composed. Molecules differ 

 from Atoms in this, that they are never 

 considered but as portions of some aggre- 

 gate. An Atom (from the Greek a priva- 

 tive, and tamno, I cut) is accounted as 

 one of the simplest particles in nature, 

 what is incapable of further division. 



MOMENTUM, or MOMENT, is the im- 

 petus, or force of a moving body. The 

 comparative momenta of bodies are in a 

 compound ratio of their quantity of mat- 

 ter and their velocity : that is, they are 

 in proportion to the products of the mat- 

 ter and velocity, when expressed in num- 

 bers. Thus a ball of four pounds weight, 

 moving uniformly at the rate of eighteen 

 feet in a second, would have double the 

 momentum, that is, it would strike 

 against an object with twice the force 

 that a ball of three pounds weight, mov- 

 ing at the rate of twelve feet per second, 

 would do ; because the first product (4 

 multiplied by 18) is double that of 3 mul- 

 tiplied by 12. Momentum is the/orce of 

 percussion. See Percussion. 



MOONSTONE. See Feldspar. 



MOTION is the passing of a body, or any 

 parts of a body, from one place to an- 

 other : we say parts of a body, because 

 in the cases of a globe turning on its axis, 

 or a wheel revolving on a pivot, the parts 

 of the body change their situation, while 

 the bodies themselves are stationary. 



MOVING POWER. See Power. 



MUSCOVY GLASS. See Mica. 



NATIVE MAGNET. See Magnet: 



NEGATIVE ELECTRICITY. See Elec- 

 tric Fluid. 



NITRIC ACID. See Azote. 



NITROGEN. See Azote. 



NON-CON D UCTORS. See Electric Fluid. 



NON-ELECTRICS. See Electricity. 



NON-ELASTIC FLUIDS. See G*and 



Liquids. 



NONIUS. See Fernier'a Scale. 

 OBLATE and OBLONG SPHEROIDS. 



See Spheroid. 



OBTUSE ANGLE. See Angle. 

 OCTAHEDRON, See Rhombus. 



OCULAR SPECTRA. See Accidental 

 Colours . 



OPACITY (Latin o/?acws,dark) , a state im- 

 pervious to light. 



ORDINATE, of an Ellipse, Parabola, and 

 Hyperbola. See Conic Sections. 



OSCILLATION (Latin, oscillatio, swing- 

 ing) is particularly applied to designate 

 the motion of a pendulum. 



, CENTRE OF. The 



centre of oscillation in a pendulous body 

 is a point in the line passing through the 

 centre of suspension, and the centre of 

 gravity. If all the matter of the body 

 could be collected in that point, any force 

 applied there would generate the same 

 angular velocity, in a given time, as the 

 same force would generate in the same 

 time, by acting similarly at the centre of 

 gravity of the pendulum, when all the 

 parts thereof are situated in their respec- 

 tive places." This point differs from the 

 Centre of Gyration, because the motion of 

 the body is produced by the gravity of its 

 own particles, whereas in Gyration, the 

 body is put in motion by some other force, 

 acting at one place only. See Centre of 

 Percussion. 



OVALS. These figures have their name 

 from their resemblance to the transverse 

 section of an egg, Latin ovum. Ellipses 

 are ovals which are formed by a fixed law, 

 but the latter is a popular term for any 

 curved figure, approaching to that shape. 

 The carpenter's oval, for example, is made 

 up of circular arcs, that unite without 

 leaving any angular appearances at their 

 junctures. 



OVALS OF DESCARTES. These, though 

 not Ellipses, are governed by a determi- 

 nate law, which constitutes them as va- 

 rieties of that curve. As in the Ellipse 

 the two lines drawn from the foci to any 

 point of the circumference vary, so that 

 the increment of one shall always be 

 equal to the simultaneous decrement of 

 the other ; so, in the Cartesian Ovals^ 

 these increments are in an invariable ra- 

 tio. " These curves may therefore be 

 defined the locus (place) of the vertex of 

 a triangle, on a given base, one of whose 

 sides bears a given ratio to the sum or 

 difference of a given line and the other 

 side." 



OVERSHOT WHEEL. See Water- 

 Wheel. 



OXYGEN, or OXYGEN GAS, is that 

 portion (something more than a fourth) 

 of the atmospheric air, which is capable 

 of supporting flame, and is essential to the. 

 respiration of animals. Oxygen is gene- 

 rally diffused throughout nature, but al- 

 ways in combination with other sub- 

 stances. United with Hydrogen Gas (In- 

 flammable Air} it forms water. It was at 

 one time supposed to be a necessary in- 

 gredient in the composition of all acids ; 

 and hence its name, which is derived: 



