CAL

CAL

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the ice. Hold the jar by the end fo covered atan angle of about 45 °, and bring that part of itwhich is at the height of the furface of thewater over the flame of a lamp or candle.When the furface begins to boil, flowly raifethe tube fo as to heat the lower part of theWater, and thus, by avoiding to give a directheat to the ice itfelf, the whole water will befeen to boil within a quarter of an inch of theice. This ebullition may be continued feveralminutes before any fenfible portion of the icewill be melted, except a fmall part of the upperportion, which is furtheft from the flame, andon account of the inclination of the jar, rifesabove the level of the bottom of the water; fothat after a time the furface of the ice is broughtby melting to be nearly parallel with that of thewater above.

If a jar be ufed, the heat of an Argand lampwill be required; and then a piece of wood orpafteboard fhould be interpoi'ed between thecold part of the jar and the glafs chimney ofthe lamp.

It has been already mentioned that light floc-culent fubftances inclofed in a ftratum of air,very materially retard the paffage of heat throughit, probably by embarraffing thofe mutual mo-tions of the hot and cold particles whereby theheat is conveyed from place to place. CountRumford alfo found a fimilar effect to takeplace in water when mixed with fubftanceseither that gave it vifcidity by diffolving in it, orthat mechanically thickened it without adtualfolution. In the former cafe ftarch was em-ployed, in the latter eider-down, as in theexperiments on the conducing power of air.The general refult of the experiments was thefollowing: when pure water took only 597feconds to heat from 3 2° to 200°, a folution of192 grains of ftarch in 2276 parts of water,placed in precifely fimilar circumftances, took1109 feconds to acquire the fame degree ofheat; and a mixture of 192 grains of eider-downdiffufed in the fame quantity of water, took 949feconds. Applying this fa£f to culinary pur-pofes it accounts for the great length of time inwhich thick half-fluid fubftances retain theirheat.

It was. alfo found that encreafmg the quan-tity of eider-down in a given ftratum of airencreafed the obftacle to the tranfmiflion ofheat ; and in like manner an additional quantityof eider-down in water encreafed the difficultywith which it heated and cooled. For whenpure water cooled 168 0 degrees in 597 feconds,

2 Manch. Mem. vol.

water with f-g of its bulk of eider-down diffufedthrough it took 763 feconds, and with fo itrequired as much as 949 feconds. In all thefecafes it was neceffary to boil the eider-downwith the water to detach the air that clings to itwith great obftinacy.

Fluids therefore acquire and lofe heat prin-cipally by the currents which always take placein them when partially heated, the warmer partafcending, and of courfe the colder part takinga contrary direction: and whatever impedesthefe motions will, proportionally to the degreeof impediment, hinder the tranfmiflion of heatthrough the fluid.

This important and beautiful law is furtherilluftrated by a mod happy application made byCount Rumford , of a remarkable peculiaritythat obtains in the fpecific gravity of water atthe lower temperatures. It was firft noticed byMr. De Luc, and afterwards examined moreparticularly by Dr. Blagden, that when waterat a high temperature is cooled, its denfityencreafes gradually (like all other bodies whenlofing heat) to about the temperature of 40°,but, from this point to 32 0 , or as much belowas it can be cooled without freezing, its denfityaElually diminijhes: fo that juft before it freezes itis confiderably lighter than at 40°. No otherfluid in nature is found to have this Angularvariation from the general law, and it is evenwanting in ftrong faline folutions. Furtherexperiments have fully confirmed this dif-covery, but with fome little modification asto the precife point of extreme denfity, thealteration being extremely fmall for two or threedegrees above and below 40°. Mr. Dalton zmakes the maximum of denfity to be 42-5, andhas added the curious fa£l that water expandsequally at any equal number of degrees bothabove and below this point.; fo that the fpecificgravity of water at 3 2°, which is 10-j- 0 below themaximum of denfity, is the fame as that ofwater at 53 0 , which is io-|- 0 above the maxi-mum,

Count Rumford applies this principle in thefollowing way. a When hot water is- pouredupon ice frozen to the bottom of a jar, thewater on the furface of the ice immediately isreduced to 3 2°; but at this heat, being lighterthan water a little warmer, it rifes into thehotter mafs above, wliilft- water at or near 40®defeends upon the' furface of the ice. Thisagain, by the further melting of a.frefh portionof the ice is reduced to 32°, when it lofes partof its fpecific gravity and rifes as the former

v. * Eflays.