EVAPORATION.

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pression in a syringe, as to ignite amadou. The latent heat of steam at 212° F. is1180°—180=1000; that of vapor, at 77°, is 1180—45=1135°; so that, in fact, thelower the temperature at which the vapor is exhaled, the greater is its latent heat, asJoseph Black and James Watt long ago proved by experiments upon distillation and thesteam engine.

Prom the preceding researches it follows, that evaporation may be effected upon twodifferent plans

1. Under the ordinary pressure of the atmosphere ; and that either,

a, by external application of heat to boilers, with a, an open fire; 6, steam; c, hotItfiuid media .

b j by evaporation with air; a, at the ordinary temperature of the atmosphere; b, bycurrents of warm air.

2. Under progressively lower degrees of pressure than the atmospheric, down toevaporation in as perfect a vacuum as can be made.

R is generally affirmed, that a thick metallic boiler obstructs the passage of the heatnrough it so much more than a thin one, as to make a considerable difference in theirre lative powers of evaporating liquids. Many years ago, I made a series of experimentsUpon this subject. Two cylindrical copper pans, of equal dimensions, were provided ;

at the metal of the one was twelve times thicker than that of the other. Each beingcharged with an equal volume of water, and placed either upon the same hot plate of'con, or immersed, to a certain depth, in a hot solution of muriate of lime, I found thate ebullition was greatly more vigorous in the thick than in the thin vessel, which Ibribed to the conducting substance up the sides, above the contact of the source ofe ht, being 12 times greater in the former case than in the latter,sol • t * le bottom of a pan, and the portions of the sides, immersed in a hot fluid medium,a u bon of caustic potash or muriate of lime, for example, be corrugated, so as to contain0 j. ° u Ue expanse of metallic surface, that pan will evaporate exactly double the quantityim Water > > n a given time, which a like pan, with smooth bottom and sides, will do„ (] niei j s cd equally deep in the same bath. If the corrugations contain three times thesta mp tallic surface, the evaporation will be threefolcf in the above circum-

or i Ces ‘ ® ut R R* e P an > "'ith the same corrugated bottom and sides, be set over a fire,'vill* 1 &tl °^' on S flue, so that the current of flame may sweep along the corrugations, itJim e ^ a P ora,e no more water from its interior than a smooth pan of like shape andI, ensi °ns placed alongside in the same flue, or over the same fire. This curious factCa Us u Ver *flefl upon models constructed with many modifications. Among others, Ipl at ea a cylindrical pan, 10 inches diameter, and 6 inches deep, to be made of tin-sel’ a vert ‘ ca l P* ale soldered across its diameter; dividing it into two equalc.fi r ' - 'uflrical compartments. One of these was smooth at the bottom, the other>. 'A fi ‘ former afforded as rapid an evaporation over the naked fire as thein e di u ’ ° Ut was outstripped by its neighbor when plunged into the heated liquid

° r a s s l la ^ ow pan of extensive surface be heated by a subjacent fire, by a liquid medium\vh eil ■ steam pipes upon its bottom ; it will give off less vapor in the same time

Cu 0ibe f IS °P en > tflau when partially covered. In the former case, the cool in-Coi >side 1 vn' r l’ rec *pl tat es by condensation a portion of the steam, and also opposestatter „ raa e mechanical resistance to the diffusion of the vaporous particles. In the°rifice ’ aS - t * le steam issues with concentrated force and velocity from the contractedllr taciri] r a ' r must °ffer less proportional resistance, upon the known hydrostaticVessels * ° the Pressure being as the areas of the respective bases, in communicating

that a f g a l[. ora t' n " by surfaces heated with ordinary steam, it must be borne in mind10 — ^ square feet will evaporate fully one pound of water per minute, or

tace mn st ~i. » r -> the same as over a naked fire; consequently the condensing sur-Jhs., wh'ich 6 e fl ua *ta extensive. Suppose that the vessel is to receive of water 2500met hy rne „ corre sponds to a boiler 5 feet long, 4 broad, and 2 deep, being 40. cubicvucli g ; nc [ 1 aur . e > an< * let there be laid over the bottom of this vessel 8 connected tubesff.cha r g e( j ^ [ n aia meter and 5 feet long, possessing therefore a surface of 5 feet square.

'"■nute. 'ph , —"“‘i mey Wl

®fluare f eet t ' a 1 ar to supply the steam for this purpose must expose a surface of 5thin copper 6 " re ’ R ^ as been proved experimentally that 10 square feet surface' 90 degrees F*!! con< ^ ense 3 lbs. of steam per minute, with a difference of temperature3 minute, th r \ l ^ e a h°ve example, 10 square feet evaporate 1 lb. of waterP : t : : 9o pp 6 ’ ( ‘mperature of the evaporating fluid being 212° F., consequentlyri re : - 30°. ! p " nn ” this evaporation the difference of the temperature is there-° r of the boiler ° nse< l uen tly the heat of the steam placed in connexion with the inte-rr cspon4i ncr to ’ 0 P 10 .^ UBe the calculated evaporation, should be, 212-j-30 = 242°,a c astic force of 53*6 inches of mercury. Were the temperature of

steam, they will cause the evaporation of half a pound of water per