23 



UNDULATORY FORCES. HEAT. [REFRACTION AND POLARISATION. 



removed a considerable distance from each other, and yet 

 the mercury of the thermometer will rise, under the cir- 

 cumstances above-named. By means of two mirrors, 

 each six feet in diameter, and about ninety feet apart, 

 we have seen a piece of meat cooked by the reflected 

 heat of a gas jet. 



Experiment 24. If the bulb of the thermometer is 

 retained, as in the last experiment, and if the mirror 

 before which the ball of iron is placed be coated with 

 lamp-black, it will be found that no heat is then reflected 

 thereby, because the blackened surface absorbs the heat, 

 and is, therefore, incapable of reflecting it. 



It will thus be seen how the power of reflecting and 

 absorbing heat may be produced in a body by merely 

 changing the character of its external surface ; and, in 

 obedience to these laws, a great variety of instructive 

 experiments may be tried. 



By way of parenthesis, we would remark, that the 

 abstract teachings of science are valuable only so far as 

 they give us an insight into natural laws, and assist us 

 to make such applications of them as may conduce to 

 intellectual progress and social benefit. In every case, 

 however, these lessons so taught can only be appreciated 

 and rendered of service in proportion to the intelligence 

 of the student. Having this object in view, we shall 

 suggest a few experiments wfiich, whilst they may still 

 further elucidate the special laws to which we now refer, 

 will also give a wide field for the exercise of the ingenuity 

 of the experimenter, so far as their application to the 

 purposes of daily life are concerned. 



Experiment 25. Instead of the iron ball mentioned in 

 Experiment 23, place a tin vessel, having one side white, 

 another polished, and one blackened, in the focus of one 

 of the mirrors retaining the thermometer in the focus 

 of the opposite mirror. The vessel is then to be filled 

 with boiling water, and must be closed at the top, so as 

 to prevent the escape of heat. It will be found, on 

 presenting the polished or white side to the mirror in 

 whose focus it stands, that the vessel will scarcely radiate 

 any heat so as to be apparent by the rise of the mercury 

 in the thermometer ; whilst the blackened side, if similarly 

 placed, will radiate considerable heat, which, by reflection 

 from the mirror, will immediately raise the temperature 

 as indicated by the thermometer. 



The employment of dark-coloured substances for ab- 

 sorbing and radiating heat is thus accounted for ; and one 

 well-known instance, where the opposite character of an 

 absorbing and reflecting or non-radiating material is often 

 used in one vessel, is that of the common tea-kettle, whose 

 lower side, in contact with the fire, absorbs heat rapidly 

 when coated with charcoal, whilst the polished surface 

 prevents the escape of heat by its non-radiating nature. 

 For similar reasons, a polished metal teapot will retain 

 heat longer than a dark stone one. If, however, the 

 latter is placed near the fire, this difference will not be 

 so evident, because the absorbing power of the stone 

 vessel will be exercised, and the heat apparently retained, 

 although it really is being continually received, and so 

 counterbalances the radiating power of the material 



Experiment 26. Place pieces of cloth of different 

 colours on snow, so that the sun's rays may shine directly 

 on them. It will be observed, that the darker the colour 

 of the cloth, the more heat will be absorbed, and the 

 deeper the cloth will sink in the melted snow. Hence, 

 dark-coloured cloths are warmer, as a clothing material, 

 than those of a light colour, because they absorb more of 

 the heating rays of the sun. 



Experiment 27. Place a tliin covering of any kind 

 over a shrub, so as to prevent the radiation of its heat 

 during the night-time. On removing it next morning, 

 no dew will be found beneath. The clouds act in pre- 

 cisely the same way, and prevent the formation of dew, 

 by impeding radiation ; hence dew is never formed during 

 cloudy weather. 



Experiment 28. Having blackened the bulb of the 

 thermometer employed in the focus of one mirror, place 

 a piece of ice in the focus of the other one, and carefully 

 adjust both, so that they shall be exactly opposite to each 

 other. If this experiment is carefully performed, the 



latent heat of the mercury of the thermometer will 

 be reflected to the opposite mirror, and will be again 

 reflected to the ice, which it will eventually melt. This is 

 a highly instructive experiment, and illustrates the action 

 of the reflected latent heat of a body when its sensible 

 heat appears to be comparatively low, or at natural 

 temperatures. 



The different effects of reflection, radiation, and ab- 

 sorption, are often observed in daily experience. Chalk 

 rocks, for instance, reflect heat ; whilst trap and other 

 dark-coloured strata absorb it rapidly. In passing be- 

 tween walls which have been painted black, the radiation 

 of heat, previously absorbed, is very great. Dark soils, 

 for similar reasons, more rapidly change their tem- 

 perature than those that are light-coloured, and the 

 atmospheric heat thus varies considerably, under such 

 circumstances, over their surfaces. Some very ingenious 

 applications of the laws of radiant heat are made for 

 domestic purposes, amongst which may be named the 

 American oven. One side of this being made of rough 

 metal, absorbs heat ; whilst the other, being polished, 

 reflects it : the conjoint action of these makes the con- 

 trivance an exceedingly valuable and economical one for 

 baking purposes. 



REFRACTION OF HEAT. 



IN our last section, we presumed that the whole of the 

 heat radiated from a body passes to any object at a dis- 

 tance from it, without sustaining any loss from its 

 absorption by the atmosphere. It is, however, found, 

 that gases, liquids, and solids prevent the passage of a 

 portion of the rays emitted from a heated body ; and some 

 substances have the power of bending rays of heat from 

 what would otherwise be their natural course. Those 

 bodies which but slightly prevent the transmission of the 

 rays of heat, have been termed " diathermanous." 



As some little difficulty may arise as to the meaning 

 of the term " refraction," we suggest a simple experi- 

 ment, which will at once illustrate its purport with respect 

 to light ; and, as heat is subject to similar laws, the 

 student will easily perceive the nature of the refraction 

 of either force. 



Experiment 29. Place a shilling in a basin, and re- 

 tire from it until the edge of the coin is but just visible. 

 If an assistant now fills the basin with water, the whole 

 of the coin will appear in sight, owing to the bending, or 

 refraction, of the rays of light. The effect is illustrated 

 in the annexed ri ft- 9 - 



engraving, where ^> 



a represents the 

 real place of the 

 coin, and b its ap- 

 parent position. 



The above is an instance of what is termed single re- 

 fraction. Light and heat also undergo double refraction 

 in some circumstances ; and the effect with respect to 

 light is easily observed, by placing a rhomb of Iceland 

 spar on a line drawn across a piece of paper. The 

 crystal, having the power of double refraction, will ex- 

 hibit the line in a double appearance. Fig. 10 repre- 

 sents this effect, where No. 1 is a piece of spar, tlirough 

 which the letters rig _ i . 



appear double ; 

 and No. 2 illus- 

 trates the sepa- 

 rated rays of light 

 passing through 

 the spar, and 

 thus, also, the 

 nature of the re- 

 fractive effect. 



Having thus illustrated the single and double refrac- 

 tion of light, we proceed to examine the analogous 

 changes which the rays of heat undergo when passing 

 through various media. 



It is necessary to bear in mind, that heat alone, and 

 heat combined with light, pass through bodies in by no 

 means the same degree. Thus the heat from a gas-light 



No. 



