CAL

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exactly in the fecondary focus, was raifed inthe following degree by difks of different metalsreturning the reflected heat of the canifter, viz.brafs ioo°; filver 90°; tinfoil 85°; fteel 70°;lead 6o°; tinfoil made bright by the affufion ofmercury y o' 3 . A difk of glafs produced only10°, and coated with wax, only 5 0 .

Roughening the tin furface of the canifterwas found in the former experiments to eticreafethe propelling power: hence the fame operationperformed on a mirror lined with tinfoil diminifhedthe reflecting power to nearly one-tenth of itsformer intenfity.

A tin refleCtor coated with a pretty thick layerof tallow had its reflecting power reduced fromtoo 0 to 8°, but again encreafed to about 40°when moft of the tallow was melted off.

Tin refleCtors were coated with ifinglafs bypouring a little of the liquefied jelly into thehollow of the mirror, and turning it round con-ftantly till the whole furface appeared to beuniformly coated with it, which dried into afmooth and nearly tranfparent membrane. Thereflecting power was immediately reduced bythe wet jelly from ioo° (the power of the cleanrefleCtor) to 31°, but as the coat of jelly gra-dually dried, the power again encreafed graduallyto about 7 2°. This {hews very ftrikingly theeffeCt of the thicknefs of the ftratum, lince indrying, it muft gradually have fhrunk, till at laftit became only a fine film of membrane. Anencreafed quantity of jelly alfo diminifhed thereflecting power, but the exaCt law of dimi-nution is not eafdy afcertainable.

As in the cafe of propelled caloric, a metalliccoating, however thin, produced the fame effeCton the refleCtor as if it had been entirely madeof the covering metal; fo when the bulb ofthe thermometer was coated with a Angle thick-nefs of gold-leaf, its abforbing power was fomuch leflened as to produce no more than abouta fifth of its natural effeCt.

It has been mentioned that when radiantheat, from whatever fource derived, is obftruCtedin its paflage by any folid body, the portionwhich is not reflected from its furface enters itsfubftance, combines with it, and then affumesa new form ; fo that the pulfatory energy whichit received from the propellent furface is totallyloft by this combination. In other words, thecaloric is not propelled through a folid body,' butunites with it in a different mode, and raifes itstemperature, till the folid fubftance becomes anew and fecondary or derivative fource of calo-rific radiations, which are again propelled fromits furface, not according to the original pro r

pellent force, but according to that which be-longs to the nature and condition of tire furfaceof the fecondary heated body.

This is alfo illuftrated by very curious experi-ments taken from the fame author. Some ofthem are the following :

Place the frame of a fereen between the can-ifter and the mirror (in the apparatus all alongdeferibed), over which ftretch any fubftance tobe the fubjeCt of the experiment. The fereenfliould be large enough totally to intercept allcommunication of heat between the canifterand the mirror. When a thin flieet of tinfoilis the fereen, the radiant heat from the canifteris fo compleatly intercepted that no fenfibleeffect is produced on the thermometer bulbbeyond it in the focus of the mirror.

Subftitute a pane of glafs to the tinfoil, andthe effect produced on the thermometer willnow be about 2o°, the full effeCt without theenclofed fereen being ioo°.

Glafs is permeated by light, which fuffershowever a certain diminution or abforption inits paflage. But this diminution is proportionallythe fame whether the light is in a diffufed orcondenfed ftate, and therefore is the famewhether, in the above apparatus, the glafs fereenbe more or lefs near to the radiating fource.Not fo with heat, for the rife of the thermometeris the greateft when the glafs fereen is clofe tothe canifter (not actually touching it) and di-minifhes in proportion as the fereen is advancedtowards the mirror. Hence it follows that thepropagation of heat through glafs is made in adifferent mode from that of light.

Caufe two fheets of tin to be painted blackon one fide only, and put them in clofe contactwith their bright fides outermoft. No fenfibleeffeCt will now be produced in the bulb of thethermometer. Invert the tin plates fo that thepainted fides are outermoft, and an effeCt of23° will follow. Remove one of the plates,and the thermometer will {hew 4°.

This experiment well illuftrates the nature ofthe propellent and abforbing powers of furfacesalready deferibed, fince the fubftance of thefereens is exaCtly the fame, and only their rela-tive pofition altered: and it is clear that theradiations only affect the furfaces of bodies,fince when the fereens are in clofe contact, theinner and touching furfaces of each produce nofenfible alteration on the propulfive power, butcommunicate their heat by the conductingpower. When one plate is removed, thoughthe aCtual thicknefs of the fereen is diminifhedone-half, one of the furfaces is now metallic,