EXPLANATION OF SCIENTIFIC TERMS. 



the same names are found in other coun- 

 tries. Topazes, in particular, are of all 

 colours. -See Quartz. 



CRYSTALLIZATION is that stateofcer. 

 tain bodies in which, when passing from 

 the fluid to the solid form, they separate 

 into portions, each portion (or crystal} 

 assuming the same determinate and an- 

 gular shape. It is a species of congela- 

 tion, but this last does not necessarily in- 

 clude the idea of separate crystals. Ice 

 was called crystal (cryxtallos) by the 

 Greeks, An account of certain crystals 

 will be found under their several names 

 in this Glossary ; but to have included all 

 that are mentioned in the volume, would 

 have been to have written a work on 

 Crystallization. 



CUBIZITE. See Analcime. 



CURVATURE. See Curve. 



CURVE. A straight (or stretched) line is 

 the measure of the shortest distance be- 

 tween two points. A curve or curved line 

 is that of which no portion, however 

 small, is straight. A crooked line may 

 be either a curved line or the junction of 

 two or more straight lines drawn in dif- 

 ferent directions. The varieties of curve 

 lines are innumerable ; that is, they have 

 different degrees of bending or CURVA- 

 TURE. The curves most generally re- 

 ferred to, beside the circle, are the Ellip- 

 sis, the Parabola, and Hyperbola, to 

 which we may add the Cycloid. 



CURVES, Evolutes and Involutes of. Let 

 a thread be wound round the curve 



C B A, fixing one end at C, and carrying 

 the other round to A. If we unwind this 

 thread, keeping it tight upon the con- 

 vexitv of the curve, its end A will de- 

 mother curve A D E, passing fur- 

 d further from the former curve 

 C B A. as the string gradually lengthens, 

 until i; reaches the point C, where it is 

 ?d to be fixed. If carried further 

 on to P, the length of the thread would 

 ie the same, and the arch EF 

 be part of a circle. The primary 

 CBA, that round which the 

 was wound, is called the Evolute, 

 a secondary curve A D E, formed 

 2 unrolling of the thread (now 



stretched out in the line C E) is termed 

 the Involute. The thread, during the 

 progress of unwinding, is, at every point, 

 a tangent to the Evolute ; thus, at the 

 point D, it is a tangent to C B A at B. 

 The Involute of a Circle is described, 

 with its use, in Mechanics, Treatise ii. 

 page 29. It is a spiral. See Spiral. 



CURVES, EQUATION OF. See Conic 

 Sections. 



CURVILINEAL, or CURVILINEAR, 

 designates figures that are bounded by 

 curve lines. Thus a Curvilinear sur- 

 face is that which can be touched by a 

 plane only in one point. See Tangential 

 Plane. A cone and a cylinder are right- 

 lined surfaces in the direction of their 

 length. A sphere and a spheroid are 

 wholly curvilinear. 



CYCLOID. If we conceive the circle 

 b n a c d to roll along the line A B ; the 

 point a being first at A, and ending at 

 B, this point (a) will describe, or pass 

 through the curved line A h p a k B, 

 which curve is termed a Cycloid. Seeing 

 that the circle rolls over the whole line, 

 it is obvious that the length A B is equal 

 to its circumference, and the general 

 properties of the curve are these : that 

 taking any point h and drawing the line 

 hg (parallel to the line on which the 

 circle rolls) to meet the circle, when in 

 the middle of its motion, at g ; and join~ 

 ing g a, the line h g is always equal in 

 length to the circular arc g n a', and die 

 portion of the cycloidal archp a is always 

 double the length of the chord g a. 

 Further, the area of the whole cycloidal 

 space A B k a p h is equal to thrice that 

 of the circle g n a c d, by which it is 

 formed. See Preliminary Treatise, p. 21 



Fig. 10. 



CYLINDER. A cylinder is a solid having 

 a circular base, and which base may be 

 considered as carried upwards in a straight 

 line, and continuing the circle in a pa- 

 rallel direction. It is a circular prism, as 

 a cone is a circular pyramid. When the 

 base is elliptical, it is a Cylindroid. When 

 the sides are perpendicular to the base, it 

 is a Right cylinder or cylindroid ; other- 

 wise it is an O blique one. 



D'ALEMBERT'S PRINCIPLE. See 

 Principle Z)' Alemberfs. 



DEAD LEVEL.See Level. 



DEGREES AND MINUTES. See 

 Angle. 



DENSITY (Latin densitas, closeness) is 

 a relative term, and denotes the com- 



