RAY 



REA 



raised before the curtain on the counter- 

 scarp of the place. A ravelin is a trian- 

 gular work resembling 1 the point of a 

 bastion, with the flanks cut off. Its use 

 before a curtain is to cover the opposite 

 flanks of the two next bastions. It is 

 used also to cover a bridge, or a gate, 

 and is always placed without the moat. 

 There are also double ravelins, that serve 

 to coyer each other: they are said to be 

 double when they are joined by a cur- 

 tain. 



RAVEN. See Convus. 



11A.UWOLFIA, in botany, so named in 

 honour of Leonhard Rauwolff, physician 

 at Augsburg 1 , a genus, of the Pentandria 

 Monogynia class and order. Natural or- 

 der of Contorts. Apoeineze, Jussieu. 

 Essential character: contorted; berry 

 succulent, two-seeded. There are four 

 species. 



RAY, in optics, a beam of light, emit- 

 ted from a radiant, or luminous body. 

 Rays are denned, by Sir Isaac Newton, to 

 be the least parts of light, whether suc- 

 cessive in the same line, or contemporary 

 in several lines. For that light consists 

 of parts of both kinds is evident,, since 

 one may stop what comes this moment 

 in any point, and let pass that which 

 comes presently after : now the least 

 light, or part of light, which may^be thus 

 stopped, he calls a ray of light. 



RAYS of the Sun. It has been found by 

 experiment, that there is a very great 

 difference in the heating power of the 

 different ra^s of light. 



It appears, from the experiments of 

 Dr. Herschel, that this heating power in- 

 creases from the middle of the spectrum 

 to the red ray, and is greatest, beyond it, 

 where the rays are invisible. Hence it 

 is inferred that the rays of light and 

 caloric nearly accompany each other, 

 and that the latter are in" different pro- 

 portions in the different coloured rays. 

 They are easily separated from each 

 other, as when the sun's rays are trans- 

 mitted through a transparent body, the 

 rays of light pass on seemingly undimin- 

 ished, but the rays of caloric are inter- 

 cepted. When the sun's rays are directed 

 to an opaque body, the rays of light are 

 reflected, and the rays of caloric are ab- 

 sorbed and retained. This is the case 

 with the light of the moon, which, 

 however much it may be concentrated, 

 gives no indication of being accompanied 

 with heat. It has also been shown, 

 that the different rays of light produce 

 different chemical effects on the metallic 

 salts and oxides. These effects increase 



on the opposite direction of the spect< 

 rum, from the heating power of the 'rays. 

 From the middle of the spectrum, to- 

 wards the violet end, they become more 

 powerful, and produce the greatest ef- 

 fect beyond the visible rays. From these 

 discoveries it appears that the solar rays 

 are of three kinds: 1. Rays which pro- 

 duce heat; 2. Rays which produce co- 

 lour; and, 3. Rays which deprive metal- 

 lic substances of their oxygen. The first 

 set of rays is in greatest abundance, or 

 are most powerful towards the red end 

 of the spectrum, and are least refracted. 

 The second set, or those which illuminate 

 objects, are most powerful in the middle 

 of the spectrum. And the third set pro- 

 duce the greatest effect towards the 

 violet end, where the rays are most re- 

 fracted. The solar rays pass through 

 transparent bodies without increasing 

 their temperature. The atmosphere, for 

 instance, receives no increase of tempe- 

 rature by transmitting the sun's rays, till 

 these rays are reflected from other bo- 

 dies, or are communicated to it by bodies 

 which have absorbed them. This is also 

 proved by the sun's rays being trans- 

 mitted through convex lenses, producing 

 a high degree of temperature when they 

 are concentrated, but giving no increase 

 of temperature to the glass itself. By 

 this method the heat which proceeds 

 from the sun can be greatly increased. 

 Indeed, the intensity of temperature pro- 

 duced in this way is equal to that of the, 

 hottest furnace. This is done, either by 

 reflecting the sun's rays from a concave 

 polished mirror, or by concentrating or 

 collecting them by the refractive power 

 of convex lenses, and directing the rays 

 thus 1 concentrated on the combustible 

 body. 



REACTION, in physiology, the re 

 sistance made by all bodies to the action 

 or impulse of others, that endeavour to 

 change its state, whether of motion 01 

 rest. 



REALGAR, in chemistry. Arsenic, 

 mineralised by sulphur, forms two ores, 

 named orpiment and realgar, the chemi- 

 cal distinction of which is not very accu- 

 rately determined. That which has been 

 named realgar is of a red colour, some- 

 times inclining to scarlet, sometimes to 

 orange. It occurs massive, disseminated, 

 and crystallised, in oblique, tetraedral, or 

 hexaedral prisms, generally small and 

 translucent, or semi-transparent, witji a 

 shining lustre. Its fracture is uneven ; it 

 is soft and brittle, and has a specific 

 gravity of 3.2, or 3.3. It exhales before 



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