HEAT OR CALORIC. 81 



u At M, in the preceding figure, a parabolic mirror is represented. 

 At B, a square glass bottle, one side of which is covered with tinfoil, 

 and another so smoked by means of a lamp, as to be covered with 

 carbon. Between the bottle and mirror, and in the focus of the lat- 

 ter, there is a bulb of a differential thermometer, protected from re- 

 ceiving any rays directly from the bottle, by a small metallic disk. 

 The bottle being filled with boiling water, it will be found that the 

 temperature in the focus, as indicated by the thermometer, is greatest 

 when the blackened surface is opposite to the mirror ; and least, when 

 the tinfoil is so situated ; the effect of the naked glass being greater 

 than the one, and less than the other." 



" When a polished brass andiron is exposed from morning till night 

 to a fire, so near as that the hand, placed on it, is scorched intolera- 

 bly in a few seconds, it does not grow hot."* 



" Fire places should be constructed of a form and materials to fa- 

 vor radiation : flues, of materials to favor the conducting process." 



12. A cork thrust into a candlestick ; some black wool pushed by 

 a knife into a slit in the cork ; some thin slices of phosphorus or sul- 

 phuret of phosphorus, laid upon the wool or wrapped in it, the focus 

 being previously ascertained by the light of a candle, will hardly ever 

 fail of success,! an ignited iron ball or a few live coals being placed 

 in the other focus. A screen of glass or metal may be held between 

 the mirrors till we are ready for the result. 



13. Fulminating mercury, or silver, or gunpowder, maybe sprink- 

 led on the wool or on charcoal, but they will by their explosion soil 

 the mirror : the effect is otherwise agreeable. 



14. Boiling water being in one focus and a delicate air, or differen- 

 tial thermometer in the other, there is an evident movement of the fluid, 

 and the glass screen being interposed, arrests and soon reverses the 

 effect. 



15. A bright metallic mirror, held before a common fire, remains 

 cold, but, if blackened by candle smoke or India ink, it becomes hot. 



16. Provide two bright tin flasks or polished metallic tea pots ; black- 

 en one with candle or lamp smoke, then pour boiling hot water from a 

 tea kettle into both ; examine the temperature, at intervals of five 

 minutes, and it will be found that for more than an hour, the bright 

 vessel will remain decidedly the hottest, and sensibly so for several 

 hours. J 



17. Fill them with cold water and place them before a bright fire ; 

 the blackened vessel will become hot, and the other will remain cold. 



* Except that a little heat passes by slow communication along the iron bar. 



t A mouse trap without the bottom, supported by the ring of a retort stand, makes a 

 good fire grate, and a sheet of copper, zinc, or iron, will protect the table from the 

 falling coals. t See the statement of experiments, page 71. 



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