RADIATION OF HEAT. 



1UGGED AND INDUSTRIAL SCHOOLS. 



curbunic acid at various temperatures, that the velocity with which a 

 body cools from the mere contact with gas (when the excess of 

 temperature of the heated body above that of the surrounding gas is 

 c instant) depends on the density and temperature of that gas; but 

 this dependence is such that the velocity of cooling remains the same 

 if those elements change in such a way that the elasticity of the gas 

 r.'inaius constant. The same chemists have also ascertained that, 

 when the elasticity of air varies in a geometrical progression, its cooling 

 power varies likewise in a geometrical progression, in such a manner, 

 that when the common ratio of the first progression is 2, that of the 

 latter is 1'366. If, instead of common air, hydrogen gas, carbonic 

 acid, or defiant gas be in contact with the heated body, the ratio of 

 the first progression being as before, that of the second is 1*301. And 

 they conclude that the cooling power of each of the last-mentioned 

 gases is nearly proportional to the square root of the elasticity of the 

 gas. (' Annales de Chimie,' yii. ; ' Annals of Philos.', xiii.) 



That the colours of bodies have some effect on the velocity of 

 radiation and on the absorption of heat has been proved by experi- 

 ments made by Dr. Stark of Edinburgh (1833). This gentleman 

 Mil-rounded the bulb of a thermometer successively with equal weights 

 of black, red, and white wool, and placed it in a glass tube, which was 

 heated to the temperature of 180 by immersion in hot water; the 

 tube was then cooled down to 50 by immersion in cold water, and the 

 several times of cooling were respectively 21, 26, and 27 minutes. On 

 winding successively black, red, and white wool about the bulb, and 

 - the temperature from 50 to 170, the times in which the 

 thermometer so surrounded acquired the latter temperature were 

 respectively 4J, 54, and 8 minutes. (Turner's ' Elements of Chemistry,' 

 Heat.) Mr. Glaisher ('Phil. Trans.' 1847) found the order of the 

 radiating power of coloured wools exposed to a clear nocturnal sky to 

 be Ijlark, green, vhite, crimton, Karlet, oranye, yellow, dart blue, liyht 

 blue, the difference between light blue and black being 1*3. 



The refrangibility of heat was first examined by Dr. (afterwards Sir 

 William) Herschel, who, having analysed by the prism, as usual, a 

 beam of solar light, and having placed a Fahrenheit's thermometer 

 successively within the fields of the different coloured rays in the 

 spectrum, found that in the violet rays the temperature was 2, and, 

 gradually increasing towards the other extremity, in the red rays it 

 was 7 above the general temperature of the apartment. He also 

 ascertained that there was a point beyond the limits of the visible red 

 ray* at which the excess of temperature was a maximum. Similar 

 observations were made about the same time by Sir Henry Englefield ; 

 and it was hence evident, not only that the calorific rays were 

 refrangible, but that the property existed in them in a higher degree 

 than in light. Dr. Herschel afterwards made a number of observations 

 <m small pencils of heat proceeding from a lighted candle, a common 

 fire, iron heated to redness, and also from iron heated to a lower 

 degree ; and he discovered that, in all these CTOOB, the calorific rays 

 were susceptible of refraction. He found however that there was some 

 ill!!, rence between the heat of the sun and that of terrestrial bodies, 

 the fonner passing more freely through the glass than the latter. 

 (' Phil Trans.', 1800.) M. Melloni has subsequently ascertained, by 

 using prism* of rock-salt (a mineral which possesses in a high degree 

 wer of transmitting heat), that heat from different sources, like 

 light of different colours, has different degrees of refrangibility. 



In the experiments on radiant heat it has been noticed, that when 

 such heat from whatever source has been once absorbed and again 

 radiated it does not retain the peculiarities of its source. But heat of 

 low refrangibility may be converted into that of higher refrangibility. 



M. Berard ascertained that when light suffers double refraction in 

 Iceland spar, the two pencils formed spectra which exhibited similar 

 properties ; in both, the calorific power differed at the two ends, and 

 existed beyond the visible red rays. Also, on polarising by reflection 

 from glass a beam of solar light, and receiving the reflected ray on a 

 Meood glass, the latter being capable of turning round till the ray 

 ceased to be reflected from it, he found that while the light was 

 reflected, the heat was also reflected, and that when no light was 

 reflected, there was no heat. The like effect was produced when, 

 inrtf-1 of a pencil of solar light a portion of radiant heat from a body 

 in >t luminous was employed, and the inference is, that the particles of 

 radiant beat are polarised by reflection, like those of light. This 

 Huliject lias however been since more completely investigated by 

 wr Forbes, (' Edinburgh Phil. Trans.', 1835.) Knoblauch, 

 (' Pugg. Anna!,' Ixxiv.) has also obtained distinct evidence of the 

 diffraction and interference of the rays of heat. 



The subject of solar radiation, or of the direct force of the sun's 

 rays, so important to the agriculturist as well as to the philosopher, 

 was treated by Mr. Daniell, in his ' Meteorological Essays,' 1823. On 

 compering a thermometer exposed to the action of the sun with one 

 which gave the mean temperature of the air in the shade, Mr. Daniell 

 observed that the power of solar radiation varies with the sun's decli- 

 ; the greatest intensity taking place in June, though the 

 greatest mean temperature of the atmosphere does not occur till July. 

 /served also that the radiation varies at different hours of the 

 day, increasing with the sun's altitude till a short time after it arrives 

 on" the meridian, and then diminishing till the evening. From the 

 observations of Major Babine at Sierra Leone, at Bahia, and at Port 

 Royal, Daniell was led to conclude that the intensity of solar radia- 



iminishes in proceeding towards the equator ; and the conclusion 

 appears to have been subsequently confirmed by the observations of 

 Scoresby and Parry, and of Richardson, in the Arctic regions. From 

 the observations of Sabiue on the mountains of Jamaica, Daniell 

 considers that this radiation increases from the surface of the earth 

 upwards. [ACTINOMETEK.] 



Since all bodies, even in vacuo, lose heat by radiation, it is easy to 

 conceive that any part of the earth's surface, when not exposed to the 

 direct action of the sun, must emit calorific rays of heat towards the 

 heavens, and thus must become cooled. This is called terrestrial 

 radiation, and the subject was particularly considered by Mr. Daniell, 

 who, from observations continued during all the months of the year, 

 found that the maximum depression of the thermometer, on account 

 of radiation, varied from 10 to 17 between midwinter and mid- 

 summer ; but that the mean depression was the least in January and 

 July, and the greatest in April. It is obvious however that numerous 

 observations are yet to be made in different regions of the earth before 

 any general theory respecting the extent and law of the variations of 

 solar and terrestrial radiations can be formed. The progress that has 

 been made on the subject of late years is noticed under METEO- 

 ROLOGY ; TEMPERATURE, TERRESTRIAL, DISTRIBUTION OP. 



RADICAL. [ROOT.] 



RADICALS, ORGANIC. [ORGANIC RADICALS.] 



RADISH. Of this vegetable there are several sorts. The Scarlet or 

 Salmon-coloured, and the Kadis rose demi-lmgue, are the best for early 

 sowing ; the latter variety is very tender and of a fine bright colour. 

 The purple, another early long-rooted variety, is sometimes sold under 

 the name of Salad Radith, the seed leaves being larger and conse- 

 quently better adapted for small salading than those of the other 

 varieties. With the colour of the preceding sorts, the Loni/ White 

 Transparent will form an agreeable contrast. Of the round or turnip- 

 rooted varieties, the Earl;/ White Turnip-rooted and the Purple Turnip- 

 rooted are the best. The Yell/no Radish succeeds in hot weather, 

 being fit for use when the earlier kinds are apt to run to seed ; and, it 

 precedes the autumn and winter varieties, of which the following is 

 the order of succession : Round Brown, White Spanish, Oblong Broirn, 

 Black Spanish, and Purple Spanish. None of these are however 

 esteemed in this country, where they are generally unknown. 



If the early white turnip-radish be sown in August, it will come into 

 use in the spring. The principal sowings should however be made in 

 January and February, and successionally till May. The winter 

 varieties require to be sown in July. In cold frosty weather the beds 

 should be protected by straw, as is commonly practised, or by such 

 other efficient means as may be at command. The covering should be 

 always dispensed with when the days are fine ; but it ought to be 

 replaced before the cold of night ensues. With careful management 

 they may be gathered nearly throughout the year, but in a hot autumn 

 it is difficult. Radish seed-pods are sometimes pickled, whilst tender, 

 and may be used instead of capers, when the latter cannot be 

 obtained. 



RADIUS (a ray, the spoke of a wheel) means the line drawn from a 

 point, considered as a centre or pole, to any point of a curve. 



RADIUS OF CURVATURE. [CURVATURE ; SURFACE.] 



RADIX (root) is applied to any number which is arbitrarily made 

 the fundamental number of any system. Thus ten is the radix of the 

 decimal system of numeration, and the radix of the common system of 

 logarithms. The term however has not acquired much fixed use, 

 though often convenient for temporary specification of the use which 

 is made of a particular number or fraction. 



RAGGED AND INDUSTRIAL SCHOOLS; REFUGES FOR 

 DESTITUTE CHILDREN. Efforts to instruct ignorant and neg- 

 lected children are not peculiar to the present time, although such 

 efforts have been of late years more systematic and sustained than 

 formerly. The Sunday schools now so numerous in this country com- 

 menced with what were in effect ragged schools. About sixty years 

 ago, Thomas Cranfield, in conjunction with a few other persons 

 connected with Surrey Chapel (Rev. Rowland Hill's), commenced a 

 " Fragment school " in the Mint, Southwark, which was followed by 

 the ^opening of other schools, similar to the present ragged schools. 

 John Pounds, a cobbler, of Portsmouth, gathered neglected children 

 into his workshop, and in this humble way was able to impart the 

 rudiments of education to about five hundred children. About 1839, 

 efforts were put forth in Bristol and in London to establish schools 

 for the neglected and outcast. Andrew Walker, an agent of the 

 London City Mission, opened schools of the kind referred to in West- 

 minster, and similar schools were soon after commenced in other parts 

 of London, till, in the course of five years, when the Ragged School 

 Union was formed (1844), there were 20 schools, with 200 voluntary 

 teachers and about 2000 scholars. 



Ragyed Schools are generally intended to impart instruction, chiefly 

 religious. At first they were usually open during the week in the 

 evening only, besides being open on Sundays. It was considered best 

 to commence the experiment with the evening school, as many of 

 the children had to provide for themselves the means of living by 

 selling articles in the streets, and most of them were unwilling to sub- 

 mit to the restraint of school hours during the day-time. This system 

 was found, in the course of a few years' experience, to work very 

 imperfectly, and paid teachers were in many cases engaged to couduc 



