RAD 



R A I 



of grapes). Paratartaric Acid. An acid 

 obtained from the grape, similar in its 

 composition to tartaric acid, with which 

 it is consequently isomeric. 



RA'CHIS ipdxiv, a ridge). This term, 

 which should be written rhachis, denotes, 

 in Botany, that form of floral axis in 

 which several pedicles, or flower-stalks, 

 are developed at short distances from 

 each other, as in grasses. 



RA'DIANT {radius, a ray). In Geo- 

 metry, a radiant is a straight line pro- 

 ceeding from a given point, or fixed 

 pole, about which it is conceived to 

 revolve. 



RA'DIANT POINT. That point from 

 which the rays of light diverge from a 

 luminous body in all directions. The 

 point in which convergent rays meet, is 

 the focus. 



RADIA'RIA {radius, a ray). A term 

 applied by Lamarck to that class of the 

 Radiata of Cuvier, which contains species 

 with a radiated form of the entire body. 

 They are distinguished into the soft and 

 gelatinous, as the medusa ; and the hard, 

 spiny, and echinodermal species, as the 

 asterias. 



RADIA'TA {radius, a ray). The de- 

 signation of animals, most of which are 

 disposed around an axis in a radiated 

 form, as the star-fish. They compose 

 one of the four great divisions of the 

 animal kingdom. 



RADIA'TION {radius, a ray). A term 

 applied to the emission of light, or of 

 heat, from the surface of a luminous or of 

 a heated body, in the form of radii, or 

 rays. In natural philosophy, whatever 

 sends forth emissions in all directions, is 

 said to radiate ; and hence we have radi- 

 ation, not only of heat and of light, but 

 also of sound. 



RADICAL {radix, a root). A term 

 applied generally to any substance which 

 is capable of combining with simple 

 bodies. A radical is termed simple, when 

 it is itself an elementary body, as chlorine 

 in hydrochloric acid ; or compound, when, 

 though itself a compound, it acts as a 

 simple body in its modes of combination, 

 as cyanogen in the cyanides. In general 

 terms, a radical, simple or compound, 

 forms an acid with hydrogen, and a salt 

 with a metal. 



RA'DICAL SIGN {radix, a root). In 

 Algebra, the radical sign is the symbol 

 ^, and denotes the extraction of a root, 

 it is a modification of the letter r, the 

 initial letter of radix, or root. See 

 Root. 



281 



RA'DICLE {radicula, a little root)' 

 The rootlet, or the rudiment of the de- 

 scending axis of plants, as it occurs in 

 the embryo. 



RA'DIUS. A rod ; a spoke of a wheel. 

 The semi-diameter of a circle, or a right 

 line drawn from the centre of a circle to 

 the circumference. 



1. Radius of Curvature. In speaking 

 of the concavity of other curves than the 

 circle, the radius of curvature at a given 

 point is the radius of a circle which has 

 the same curvature as the curve has at 

 that point. 



2. Radius Vector. The radius vector 

 is a right line drawn from the centre of 

 force (in any curve, on which a body is 

 supposed to move by centripetal force) 

 to that point of the curve where the body 

 is supposed to be. It is a general radius 

 to the curve, and has the addition of 

 Vector, or carrier, because it is imagined 

 to carry forward the body to which it is 

 attached. The earth, for example, 

 moves in an elliptic orbit, of which the 

 sun (the centre of force) is in one of the 

 foci; and, by consequence, the radius 

 vector is continually increasing in length 

 during her course from the perihelion to 

 the aphelion, and decreasing in the same 

 proportion in the progress of her return.. 



RA'DIX. Literally, a root ; and, 

 hence, applied, in arithmetic, to the fun- 

 damental of any system of numeration. 

 Thus, 10 is the radix of the decimal sys- 

 tem, and of the common system of loga- 

 rithms. 



RAIN. When the temperature of the 

 air is above 32°, or the freezing point, the 

 water separated from the air falls to the 

 earth in the state of rain. It is generally 

 thought that the precipitation of water 

 from the atmosphere is the effect of the 

 mingling together of currents of warm 

 and of cold air. 



RAINBOW. A meteor in form of a 

 party-coloured arch, or semicircle, ex- 

 hibited only at the time when it rains. 

 It is always seen in that point of the 

 heavens which is opposite to the sun, 

 and is occasioned by the refraction and 

 reflection of his rays in the drops of fall- 

 ing rain. The secondary rainbow may 

 be explained by supposing a double re- 

 fraction and reflection of the sun's 

 beams. 



RAIN GAUGE. An instrument for 

 ascertaining the comparative quantity of 

 rain which falls in different places and in 

 different seasons. It is sometimes called 

 pluviometer. 



