CAL

CAL

lavage date to kindle fire, is well known to bethe friction of two fticks or other combuftiblefubftance.

In exciting heat by friction it is obvious thatthe hardeft bodies are thofe which, ceteris pari-bus, would produce the greateft mechanical im-pediment, and therefore ought to give out themolt heat. Thus a hard twilled cord drawnviolently through the hand will produce a morefevere burn than the fame loofely twilled. Butthe hardeft bodies, fuch as metals and Hones,are alio the belt conductors of heat, and henceit may often happen that a given intenfity ofheat will be fooner accumulated in a foft fub-ftance of difficult conducing power than in thehardeft metal.

Mr. Piftet found in fome experiments onfridlion r that much more heat was produced bythe fridlion of wood on wood, than of brafs onbrafs, and the moll of all when a few filamentsof cotton were interpofed.

The molt accurate, and from the large fcalein which they were carried on, the moll ftrikingexperiments on the heat arifing from fridtion,are thofe which Count Rumford z made at theordnance-foundery at Munich , with the ma-chinery employed to bore cannon. The leadingparticulars are the following : a cylindrical pieceof brafs (being a wafte portion of a fix-pounderbrafs cannon) weighing about 113 lb. was fixedin the boring machine, and a blunt fteel borerwas introduced and forcibly prefled again ft theinlide with a preffure equal to the weight of1000 lb. The cylinder itfelf was then made torevolve on its axis, and thus to work againft thebore (as in the ufual method of hollowing can-non) by a two-horfe power. The cylinder waswrapped up in flannel to retard the difperfionof heat. In 30 minutes (or after 960 revo-lutions) a thermometer laid in contadl with thecylinder in a place hollowed out for its recep-tion, rofe from 60° to 130°, l'o that the tem-perature of at lead 113 lb. of metal had beenraifed 70 degrees, and this quantity of heatwould have been fufficient to boil 5 lb. of ice-cold water. To prevent all poffibility of anyportion of this heat having been furnifhed bythe air, Count Rumford repeated the experimentwith an ingenious contrivance for caufing theborer to work in vacuo, and an equal effedt wasproduced as in the former cafe. The thirdexperiment was Hill more ftriking. The wholeapparatus was inclofed in a water-tight box, andcovered with about 2\ wine gallons of water at60°, and the .borer fet in motion. The heat

1 Effai fur le Feu.

thereby generated was communicated to the-water, the temperature of which rofe in onehour from 6o° to 107°; in half an hour moreto 142 0 ; in another half-hour to 178°; and intwenty minutes more it actually boiled. Thustherefore 2% gallons of water were heated 152degrees by ftrong friction for two hours andtwenty minutes, befides an equal rife in thetemperature of the brafs cylinder, weighing113.331b.; of 36£ cubic inches of iron andfteel in the borer and other apparatus, befideswhat was difperfed in the air and communicatedto the box. By very fair eftimation, the wholeheat thus generated was more than equal tohave raifed to the boiling point 26.58 lbs. aver-dupois of water at 3 2°: or (by inference fromfome of Dr. Crawford’s experiments) it wouldhave required the united effedt of more thannine wax candles three-quarters of an inch indiameter, and applied in the mod advantageousmanner, to have heated as much water to thefame point.

In the above experiments the queftion arifeswhence this large acceffion of heat could havebeen derived ? It is obvioufly generated by thefriction, and apparently in a very uniform man-ner ; but no other fenfible effect is produced bythe borer than that of turning out a large quan-tity of fragments of brafs (fomewhat refemblingwood fhavings in form) and poffibly, by the.great preffure, a flight condcnfation of the re-maining hollow cylinder of metal. But theturnings of the brafs come out quite bright andmetallic, and have undergone no obvious chemi-cal alteration whereby caloric could have beenelicited, and therefore we muft either accountfor the heat by the diminilhed capacity for cal-oric of the brafs turnings, or acknowledge fric-tion to be a fource of heat entirely diltinct fromany we have hitherto examined. To afcertainthe capacity of the brafs turnings, Count Rum-ford added 1016.5 g ra i ns °f them at 210°, to459° grains of water at 59.5 0 , and the commonheat after mixture was 63°, whence by Craw-ford’s rule the fpecific caloric of the brafs turn-ings is .no. An equal quantity of thin flipsof gun-metal cut out by a fine faw was examined,under fimilar circumftances ; and the commontemperature and of courfe the fpecific heat werefound precifely the fame. Hence (even allowinga little higher fpecific caloric in cajl brafs overthe turnings from the borer) the utmoft effedtof this could only account for a very minuteportion of the adtual heat produced by thefriction, nor is it probable or even poffible that* Phil. Tranh 1798.