R. 



RADIATION OF HEAT. 



E. 



Ris one of the vibrating letters called liquids. It is formed at the 

 back of the palate, and is on this account more nearly related to 

 the liquid I than to n or m. For the various forms of the alphabetical 

 symbol see ALPHABET. It is convertible 

 1, with I. See that letter. 

 ~2, with n. See that letter. 



3. with rn at the end of words. See K. 



4. with s. See S. 



5. It is apt to place itself at one time before, at another after a 

 vowel. Thus in Greek Kf>OKoSei\os or Kopno8fi\os, Kparos or Kapros. 

 So the English words red, run, are changed in the Dorsetshire dialect 

 to hird, kirn. Again, brid is an old orthography of bird, and the town 

 Bridlington is pronounced Burl* nylon. 



6. The letter r, in the neighbourhood of several consonants, is apt to 

 disappear from words. Thus the German sprcch-en is in English apeal-, 

 our word icorld is in German welt. 



7. In one language a word is found with an initial r, when in other 

 allied languages there occur at the beginning two consonants, as In-, fr, 

 ?rr. Thus in Greek we have poSov, pijyvvfju, pffa, connected with which 

 are the forms /3j>o5oc, frango, Latin, and break, English; and, thirdly, 

 the English words icreak, icoi-k, vrouylil. 



8. The letter r is at times confounded with u>. Thus it is not a 

 very rare variety of articulation that rubbish is pronounced wubbish. 



9. More particularly when a word ends in a w, or even a vowel, it is 

 not uncommon to pronounce an r, especially if the next word begins 

 with a vowel. The London vulgarism, winder, filler, for window, 



. is an example, nor need the philologist be ashamed to treat of 

 such cases, which are as worthy of consideration as any dialect of the 

 Greek tongue. 



RABBI (/a/30i, <1 2'^), a title of respect, similar in meaning to our 

 word master or teacher, which was given to the teachers of the 

 Jewish law by their disciples and the people in the time of Christ. 

 (Matt., xxiii. 7.) The title was often given to Christ by his disciples. 

 (Mutt., xxvi. 25, 49 ; Mark, ix. 5 ; xi. 21 ; John, i. 38 ; iv. 31.) It is 

 doubtful when it was first introduced. It is admitted by Jewish 

 writers that it was not in use before the time of Hillel of Babylon, 

 who lived in the let century before the Christian era; and it was 

 perhaps first introduced into the Jewish schools about the time of 

 Christ. The word was originally used in three forms : Rat> (3"^), as 

 the lowest degree of honour; Rabbi ( 1> 2"2), of higher dignity; and 

 Rabban (713"]), or Rabbmi (pa00ov[), which was the most honourable 



of all. The title of Rabboni is given to Christ on two occasions in the 

 evangelists. (Mark, x. 51 ; John, xx. 1C.) 



The title of rabbi has continued in use among the Jews in modern 

 times. The term rabbinical has been given to all the Jewish writings 

 composed after the Christian era. 



(Buxtorf, Lexic. Chald. Rab. Talmud., 2176 ; Lightfoot, Hor. Hebr. 

 ad Math., xxiii. 7 ; Hill, De Hebraeor. Rubbinii, Jen. 1741 ; Winer, 

 rttrbuck, art. Rabbi.) 



RABDOLOGY. [NAPIER'S BONES.] 



RACEMIC ACID. [TAHTARIC ACID.] 



RACHITIS. [RICKETS.] 



RACK. [ARRACK.] 



KAi'K. [TORTURE.] 



RADIATION OF HEAT is a motion of its particles in rectilinear 

 directions, diverging every way from a heated body, either luminous or 

 not; and it is imagined to arise from the existence of a strongly repul- 

 sive power by which the particles are made to recede from each other 

 with great velocity. 



The intensity of heat thus emitted from a point of radiation is 

 obviously the same at equal distances from the point; and at unequal 

 distances, it is inversely proportional to the squares of the distances. 

 The radiating particles falling upon the surfaces of any bodies in the 

 vicinity of that from which they emanate, are, according to the nature 

 of those bodies, absorbed in them, or transmitted through them, or 

 again they may be reflected from them ; and, in the two last cases, the 

 radiant heat, as it is called, appears to suffer modifications analogous to 

 those which, in like circumstances, take place hi light. According to 

 M. Prevost (' Essai sur la' Calorique Hnyonnante,' 1809), the radiation 

 of heat is a process which is perpetually going on among all the bodies 

 in nature; those which are of equal temperature mutually inter- 

 changing equal quantities of caloric ; but, with respect to two bodies 

 which are unequally heated, that which has the greatest quantity 

 sends forth emanations in greater abundance^than the other ; the differ- 

 ence however diminishing as both bodies approach to an equality of 

 temperature. 



The particles of heat (calorific particles) appear to move with perfect 

 freedom through a vacuum, and to be impeded, but in an insensible 

 degree, in their progress through air or any of the gases ; they are 



ARTS Ain> SCI. DIV. VOL. VI. 



also found capable of being transmitted, though in small quantities 

 through transparent media of the denser kinds, as glass, rock-crystal, 

 &c.; and in passing through air they produce no sensible effect on 

 the temperature of the latter. If a body be heated to any degree of 

 temperature, and be placed in an absolute vacuum, it is evident that, 

 in consequence of the repulsive power above mentioned, the heat must 

 at length be entirely dissipated by the radiation merely ; and if the 

 body be placed in any fluid, it may be readily concluded that the 

 abstraction of the heat must be influenced by the conducting power of 

 the fluid. 



The first direct experiments which appear to have been made on the 

 radiation and reflection of heat are those of Mariotte, and an account 

 of them is given in the ' Mdmoires de 1'Acad.,' 1682. He caused the 

 heat of a fire to fall on the surface of a concave mirror, and observed 

 that it was concentrated in the focus of the latter ; and on placing a 

 plate of glass between the fire and the mirror, he perceived that the 

 rays of heat were intercepted. The subject does not appear to have 

 been much attended to till about the middle of the 18th century, 

 when Lambert, in his ' Photometria' (1760), states that, on placing a 

 large glass lens before a fire, the heat was scarcely sensible at the focus, 

 while the reflected heat of burning charcoal set fire to combustibles at 

 a considerable distance ; and Scheele, in his treatise on air and fire, 

 which appears to have been written in 1775, describes radiant heat as 

 differing from ordinary heat by disseminating itself iu right lines 

 whose directions are not changed by the agitations of the air, and by 

 being reflected from polished metallic mirrors, while it is absorbed in 

 those of glass, and in the others when their surfaces are blackened. 



The experiments of Scheele were varied by MM. Saussure and 

 Pictet, who by an appropriate apparatus endeavoured to ascertain the 

 laws of the radiation of caloric. They employed for this purpose two 

 concave mirrors of polished tin, in the focus of one of which was 

 placed the bulb of a Reaumur's thermometer, and in that of the other 

 a ball of iron heated below the degree necessary to render it luminous ; 

 and, by a comparison of the height of the mercury iu the thermometer 

 with that in one which was placed out of the focus, but at an equal 

 distance from the iron, the difference was found to be equal to about 

 8 degrees. M. Pictet employed also, in place of the heated ball, a 

 glass flask containing boiling water, in order to avoid the risk of any 

 light being combined with the heat ; and the effect produced on the 

 thermometer sufficiently proved that the calorific rays exist in- 

 dependently of those of light. (Pictet, 'Essai sin- le Feu,' 1790.) In 

 pursuance of the experiments of Scheele with a blackened mirror, 

 M. Pictet covered with lampblack the bulb of the thermometer in the 

 focus of one of his mirrors, and found that the concentrated heat from 

 the flask, which when the bulb was bright raised the mercury 2 

 degrees, now raised it 4J degrees. The same experimenter placed a 

 flask of snow in the focus of one metallic reflector, and a thermometer 

 in that of another ; and the observed fall of the mercury was then con- 

 sidered as an indication that cold is susceptible of radiation and reflec- 

 tion like heat. The last experiment belongs however to an earlier 

 date, as may be seen by reference to a letter from Oldenburg, dated 

 1665, in Boyle's Works (fo. 1744). 



The nature of the apparatus, and the experiments made by Leslie on 

 the radiation of heat, are described under HEAT. From these it 

 appeared that the amount of radiation from a polished surface was only 

 about one eighth of that which took place from a surface without 

 polish ; and hence it was evident that the velocity of radiation depends 

 more on the surface than on the nature of the radiating body. In 

 making experiments respecting the absorbing power of substances, 

 Leslie found that when the heat from the canister was suffered to fall 

 on the glass bulb of the differential thermometer, the quantity received 

 by the latter, if covered with tin-foil having a polish, was only one- 

 fifth of that which it received in its ordinary state. 



Leslie also repeated the experiments of Pictet for determining what 

 was called the radiation of cold ; and filling his canister with ice or 

 snow, he found that the cold apparently emitted from the varnished 

 side was the greatest, and that from the polished side the least ; ho 

 observed also that the cold, like the radiant heat, varied with changes 

 in the absorbent power of the thermometer and of the surface of the 

 mirror. The mercury in the thermometer fell more when the mirror 

 was of polished metal than when of glass ; it also fell more when the 

 bulb was in its ordinary state than when covered with a polished tin- 

 foil. (' Inquiry into the Nature of Heat,' 1801.) These circumstances, 

 which seemed at one time to favour the opinion that cold had a 

 material existence like heat, have received an explanation from M. 

 Prevost of Geneva. (' Journal de Physique,' xxxviii.) This philo- 

 sopher observes that a mass of ice or snow may send out radiant heat 

 less in abundance than that which is emitted from other bodies in its 

 vicinity ; and this comparatively small quantity of heat, when reflected 

 from one mirror to the other, and from the surface of the latter to the 



3 M 



