CAL

CAL

( 214 )

“ The capacity of the vapour of condenfiblefluids appears to be ftill more encreafed by ex-pan (ion than that of the permanent gaffes, andconfequently the vaporization of liquids requiresmore heat when the vapour exifts in an expandedthan in a condenfed date. This circumftancethrows a great obftacle to the plan of diftillationin vacuo economically confidered, which hasbeen propofed by fome ingenious perfons, andwhich at firft fight would appear to promifemany advantages. It is undoubted that a liquidwill evaporate and boil in vacuo.at a much lowertemperature than in the free air, but the con-ftitution of vapour in this fituation requires amuch greater quantity of combined caloric thanwhen under external preff'ure; and confequently,the advantage gained by boiling at a lower tem-perature is counterbalanced by the larger quan-tity of fuel required to furnifh the additionaldofe of combined or latent caloric demanded bythe vapour when uncomprefled. The experi-ments of Mr. Watt firft proved the truth ofthis pofition. r A fmall ftill was half filled withwater and very clofely luted to the worm-pipeof the cooler, which was alfo furnifhed with anair-tight ftop-cock at the lower end throughwhich the diftilled liquor drops in common dif-tillation. To empty the whole apparatus of air,the water in the ftill was made to boil violentlyand the fteam to pafs through the whole of theworm-pipe uncondenfed, no water being putinto the tub of the cooler. This expelled nearlyall the common air contained in the veflels, andthe ftop-cock was immediately clofed artd thefire removed. The fteam then condenfed, andnearly a perfect vacuum was left in the veflels.The cooler was then filled with cold water andthe ftill very flowly heated, the ftop-cock at thebottom remaining all the while clofed. Thewater diftilled perfectly well in this vacuum ata very low temperature, and the heat of thefteam was found to be only ioo°inflead of 212°as in common boiling, fo that the veflels fcarcelyfelt hot to the hand. But by eftimating theheat acquired by the water of the cooling-tubduring the eondenfation of the fleam, it wasfound to be fo much more than what wouldhave been produced by common fleam, that bycalculations fimilar to thofe already mentioned,the latent heat of this uncomprefled fteam muftbe reckoned at 1048, inftead of about 840, theheat of common fleam. With a more perfectvacuum, Mr. Watt found that water woulddiflil very well at 70°, but then the latent heatof its vapour approaches to 1300.

r Black’s Lectures, p, jS?8.

From the refult of fome experiments devifedwith great ingenuity, Mr. Dalton * concludesthat the heat and cold produced by the mechani-cal rarefaction and eondenfation of air is in factmuch more than is indicated by the thermometer(which rifes or falls only a very few degrees),on account of the fmall lize of the veflels ufedin the experiments, anti the comparative fud-dennefs with which the equilibrium is reftoredfrom without. The rapidity of the change ofthe thermometer in thefe cafes compared to thetime required for an equal change, when theinftrument rifes or falls a few degrees in theopen air, feems to confirm the conjecture of agreat excefs of heat or cold acting fuddenly,and as fpeedily counteracted; and, among others,the following experiments appear to prove it.A fmall graduated glafs tube, io inches long,and with a bore only X ' T of an inch diameter,was clofed at the lower end, and a fhort columnof mercury introduced a little way down theupper or open end. The bore of the tube beingfo fmall, the mercury remained where it wasintroduced without dropping down, thus fepa-rating from the external air that portion of airwhich was contained in the part of the tubebelow it, but by its rife or fall it would readilyindicate the expanfion or contraction of thisconfined portion of air. The tube was intro-duced under a glafs receiver of an air pump, towhich a condenfer was alfo attached. Thereceiver therefore would contain two very un-equal portions of air, but under the fame cir-cumflances of preflure, the fmaller portion beingthat very minute quantity intercepted in thegiafs tube between the mercury and the clofedextremity, and the larger portion being all thereft of the contents of the receiver. Now, asthe fmaller portion is clofely contiguous to athick wall of glafs, and is only -ps of an inch indiameter, all the changes of temperature withinit, arifing from rarefaction or eondenfationaltering its capacity for caloric, may fairly befuppofed to be almofl inflantly counteracted,and the equilibrium reftored by the contiguousparts ; but in the larger portion, which includesby far the greater part of a large receiver, a veryfenfible time muft elapfe before the equilibriumcan be entirely reftored. This time, the authorreckons at about 10 feconds.

In the firft experiment, the denfity of thewhole air within the receiver was doubled by acondenfer, and of courfe the fmall column ofmercury was prefled down in the fmall tube,communicating to the air beneath the fame

5 Manch. Mem. vol v.