459 delations of Air, Heat, and Cold. 



forth its heat th.c most copiously, it will also, when opposed 

 to the tide, aircst with entire ellicacy ihe afiluent wave ; 

 and if, on the other hand, a suriace of metal sparingly 

 parts with its heat, it in like manner detains only a fjoiall 

 share of each appulse, and reflects all the rest. The power 

 t)f superficial al>Sf>rption and that of reflection are therefore 

 exactly contrasted, and the one always supplies the defi- 

 ciency of the other. The naked bulb of a thermometer 

 held near a goblet full of boiline; water, will mark a very- 

 sensible afHux of heat ; but if it Ije gilt or covered with tin- 

 foil, it will scarcely seem at all affected. For the same 

 reason, llse hand cased in a glove of burnished metal may 

 -Approach the fire with impunity, since the vehernent pulsa- 

 tions of heat are mostly driven back, or turned aside from 

 their attack. A sheet of paper, opposed to the aerial tide, 

 will absorb the whole impression, a pane of glass will re- 

 pel about one-tenth part, while a plate of polished silver 

 •will reflect nine-tenths of the heat, detaining only the re- 

 ■ inaining tenth. But if the metallic plate be covered with a 

 pellicle of the SOOOih of an inch in thickness, out of 10 

 parts of heat no more than three will be reflected, the rest 

 being now absorbed ; and by applying successively oilier 

 pellicles, till 3 coat eipial to the 500lh of an inch in thicks 

 ness has been formed, the quantity of reflection will gra- 

 dually become insensible. The power of a metallic specu- 

 lum in concentrating at its focus the pulses of heat or cnlj 

 13 hence very striking, while the corresponding effects of ^ 

 glass mirror seem to be extremely feeble. 



" The very different powers of a vitreous and of a metallic 

 surface in propagating or absorbing the pulsations of heat, 

 are well contrasted by an exj^erim.cnt of the simplest and 

 easiest kind. Let a small pane of glass about four inches 

 square have ona of its sides half covered with smooth tin- 

 foil ; or, what is more elegant, let a small square of thin~ 

 mica have one side plated half over with silver leaf. On 

 holding the partly covered surface of the glass or mica op- 

 posite and very near the fire for the space of a few seconds, 

 and then passinsr the finger lightly over the posterior sur- 

 face, scarcely any warmth is perceptible under the metallic 

 fheath, but an intense degree of heat will be felt behind the 

 r.aked portion of the plate. Again, reversing its position 

 pnd exjiosing the uncovered side to the fire, an opposite 

 though less marked effect is observed ; the coat of metal 

 will become sensibly hotter than the adjacent naked space; 

 because the heat absorbed along the interior surface being 

 afterwards more feebly discharged from the tin or silver leaf. 



