CAL

CAL

( 202 )

Mercury - - - - 38.6

Olive and linfeed oil - 37.8

Oil of camomile - - 37.2

Water faturated with fait 34.9

Alcohol - _ _ _ _

33-7

151 5

15:

15:

15

14

J 3-4

*311.610.9

Mercury is however the moil uniform in itsexpanfion for equal quantities of heat, of anyfluid known, and doubtlefs chiefly for the reafonalready mentioned, namely, that its boiling pointis higher than other fluids. Hence its peculiaradvantage in the confirmation of thermometers,under which article the expanfion of fluids willbe further noticed.

The expanfion of folids is much lefs thanthat of fluids or gaffes, and is fo fmall that itcannot be accurately obferved without a micro-meter, and mechanical contrivances of unequallevers for fucceflively multiplying the effect.Thefe machines are called pyrometers, and thatinvented by Mr. Smeaton is efteemed one ofthe molt perfect and accurate. A pyrometerof platina has been attempted by Guyton, h tomeafure the higher degrees of heat. It confiftsof a bar of this metal enclofed in a groove ofhighly burnt Clay, and with a complicated indexat one end, whereby an expanfion amountingto not more than about a five-thoufandth of aninch is clearly afcertained. The expanfion offolids appears to be pretty equable for equalquantities of heat.

Gafles and gafiform vapors expand very largelyby heat, much more fo than folids or fluids.The dilatation of air is fhewn very ftrikingly bythe manometer or air thermometer as it is fome-times termed, which confifts Amply in confiningair in a bulb of glafs by a column of colouredliquor that fills the tube communicating with it.On changing the temperature of the air in thebulb in any degree, however flight, the con-fequent expanfion or contraction is immediate-ly feen by a confiderable fall or rife of thecoloured liquor. But as the extreme elafticityof air caufes it alfo to be enlarged or contractedby a diminifhed or encreafed preflure of theatmofphere, a barometrical correction on thisaccount is always requifite in manometricalexperiments.

In all chemical experiments on which thequantity of gafles is to be eftimated, the tem-perature at the time of meafuring them muft betaken into the account, and proper correctionsmade when this exceeds or falls fhort of theftandard of comparifon. Thus 1000 parts ofcommon air at 3 2°, expand to 1058 at 6o°, and

to 1104 at 82°.

A great number of experiments have beenmade by different perfons to afcertain the expan-fion of air by heat, as it is a fubjeCt intimatelyconneHed with many phyfical obfervations, andin particular with all barometrical experiments.When the natural atmofphere is examined, thequantity of water which it holds in folution molleffentially modifies the expanfibility, and in lowtemperatures much of the aqueous vapour whichit held in folution is deftroyed and feparated ina liquid form. This vapour may be almoflentirely feparated from air by feveral chemicalmeans, fuch as putting the air in contaCl withfulphuric acid, or folid potafh, which have foftrong an affinity with water as to extraCl itfrom the air that holds it in folution.

From late experiments by Mr. Dalton inEngland, and M. Gay Luflac in France , bothof which bear the mark of great accuracy, andcorrefpond very clofely with each other, thevery important and convenient inference is de-duced, that all gaffes expand equally by equaldegrees of heat, provided they be free fromcondenfable vapour at the temperatures at whichthe experiments are made. Hence a table ofthe expanfion of any one gas will apply to allthe reft.

Gaffes alfo appear to expand pretty equallyfor equal degrees of heat, and not in an en creat-ing ratio, like water or other denfe fluids. Indeedthe thermometer indicates a contrary rate ofexpanfion, that is, one that regularly diminifhesas the temperature encreafes; but Mr. Daltonvery acutely fuggefts that this anomaly is in allprobability only owing to the inaccuracy of thethermometer itfelf, which from its being dividedinto equal degrees, is not a perfectly accuratemeafure of equal expanfions in the higher tem-peratures, for the reafon already mentioned:and the effect of the inaccuracy in this inftru-ment is to give to a really equal expanfion theappearance of a diminifhing ratio. The experi-ments on the expanfion of aeriform bodies willbe given under the article Gas.

The only apparent exceptions to the generallaw of the expanfion of all bodies by heat are,the regular contraction of baked clay in en-creafing degrees of heat, on the one hand, andon the other the great expanfion which manyliquids undergo in the ad of congelation. Withregard to the former it will be further explainedunder the article Pyrometer (Wedge-wood's.)

The degree of expanfion which water under-goes by congelation is very confiderable, andits force is one of the molt (if not the molt)

k An. Chim. tom. 46-