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ui winch it is lramerfed is fo fmall, that in moilinftances the temperature of the medium maybe confidered as Stationary.

Though the radiation of caloric generallytakes place in the medium of the atmofphere,and certainly neither in denfe fluids nor infolids, it does not appear that any medium isneceffary to convey the radiations of heat. Cer-tain it is that according to apparently accurateexperiments, caloric does pafs directly throughthe torricellian vacuum, though it is phyficallyimpoflible to confine a thermometer bulb withina vacuum without fome neceffary communicationwith the external apparatus through the Itemof the inftrument, fo as to be able to conveyheat by a different path than that of radiation.It appears that caloric is capable of radiatingthrough a vacuum, when the Tides of the veffelthat forms the vacuum have a higher tem-perature than the vacuum itfelf, or than anyfubftance placed in it; or vice verfa, whenthe enclofed fubftance is hotter than the fidesof the confining veffel. Count Rumford ’s ex-periments on the comparative conducting powerof air and a vacuum are much to the point. r

Count Rumford very judicioufly rejected thecommon air-pump receivers, as it is well knownthat none will enfure a perfeCt vacuum, bothon account of the impoflibility of rigorouflyexhaufting all the air, and becaufe of the wateryvapour which always arifes from the moiftureand the oil of the apparatus, and which, likerarefied air, is a conductor of heat. He there-fore employed the torricellian or barometicalvacuum, and by ingenious management fol-dered the Item of a thermometer tube intothat of a glafs bulb, which formed the vacuum,fo that after filling with mercury, inverting thetube, and fufing together with a blowpipe thetwo Items, a compleat infulated vacuum wasleft in the larger glafs ball, with the bulb of afine thermometer enclofed in the centre.

This apparatus was then immerfed in warmwater of a certain temperature to become uni-formly heated, and then taken out and fuddenlyplunged in water at the boiling point, and keptat this heat during the whole of the experiment.The confequence was a gradual but flow rife ofthe enclofed thermometer during the whole timeof immerfion. The minute particulars neednot be related, it is fufficient to obferve thatfrom 72.5 0 it rofe in 1 min. 30 fee. to 81.5, in4 min. to 133 0 , and in 5 minutes to 141°. Onadmitting air to the fame apparatus and repeat-ing the immerfion, the thermometer now rofe

* Ph, Tr;

to the lame points in a much lhorter time,requiring refpeftively only 45 fee., 2 min. 10 fee.,and 2 min, 40 fee.

The fame kind of experiments were repeatedwith a more delicate apparatus, in which thecommunication between the thermometer andthe external part of the apparatus was only afilk thread, and Hill the external heat arrived atthe enclofed thermometer through the vacuumin progreflive fteps, and by reversing the experi-ment the acquired heat efcaped from the ther-mometer to the external air.

By a confiderable agreement of feveral experi-ments, Count Rumford concludes that the con-ducting power of a vacuum compared with thatof the fame extent of air perfectly at reft, isas 702 to 1000 nearly; for in the above experi-ments the quantity of heat communicated beingequal, the intenfity of the communication isinverfely as the times.

But, as will be feen in the next feCtion, it isdifficult to make an eftimate of the true con-ducting power of air diftinCt from that of theintenfity with which caloric radiates throughthis medium, and therefore thefe experimentsare only fatisfaCtory as to the leading faCts thatcaloric will pafs through a vacuum, though withlefs rapidity than through air. It would be aninterefting fubjeCt for experiment (though noteafily managed) to afeertain how much thenature of the propelling furface affeCts the radi-ation of heat through a torricellian vacuum.

Of Conducted Caloric.

All bodies in nature are pervious to caloric,for no fubftance has ever been difeovered whichwill for any length of time retain or confinethat modification of heat and of cold which isfenfible to the feelings and to the thermometer..But caloric is found to radiate through no othermedium than the aeriform, confequently itspaffage through folids and liquids muft be per-formed in a different manner; and even throughair it does not entirely pafs by radiation, fince-the ftate of the air as to denfity, moifture*motion, &c. is found to make a very decideddifference in the eafe with which heat pervadesit.

When heat combines with any fubftance, thefirft effedt is to expand it in every dimenfion,and this law is fo univerfal as not to have aAngle proper exception. When any portion ofa folid body is expanded, the only effedb can beto produce a mutual receffion of the particlesadded on, and not a difplacement of their com-parative juxta-pofition. If a perpendicular bqrf. 1286..