EVAPORATION. 



In such case the evaporation takes place, not 

 only from the surface, but from every point in 

 the interior of the vessel. This indicates that 

 the evaporating force of the water gained from 

 the increase of heat has become greater than 

 the pressure of the atmosphere, which is suffi- 

 cient to sustain a column of mercury thirty 

 inches high in the barometer. If the pressure 

 of the atmosphere be removed by the air-purnp, 

 the turbulent evaporation or boiling of water 

 will take place at a much lower temperature 

 than 212, even down to the freezing point. 

 But under the common pressure of the atmo- 

 sphere, and below the temperature at which 

 water boils, evaporation goes on quietly and 

 slowly. In deep mines, which descend below 

 the level of the sea, water requires a greater 

 heat than 212 to make it boil. But on high 

 mountains, or districts rising far above the 

 level of the sea, the pressure of the air is less- 

 ened, and boiling takes place, as in the air- 

 pump, at lower degrees. 



The vapours exhaled from a liquid at any 

 temperature contain more heat than the fluid 

 from which they sprung; and they cease to 

 form whenever the supply of heat into the 

 liquid is stopped. Nevertheless, a thermome- 

 ter held in the steam proceeding from hot water 

 rises no higher than when placed in the water 

 itself. The additional heat, therefore, contained 

 by the vapour, is in a latent or concealed state, 

 and does not become sensible to the thermome- 

 ter until the vapour condenses. Any quantity 

 of water requires, for its conversion into vapour 

 or steam, five and a half times as much heat 

 as is sufficient to heat it from the freezing point 

 of 32 to the boiling point of 212. The quan- 

 tity of heat absorbed by one volume of water 

 in its conversion into steam, is about 1000 

 Fahrenheit; it would be adequate to heat 1000 

 volumes of water one degree of the same scale ; 

 or to raise one volume of boiling water, con- 

 fined in a non-conducting vessel, to 1180. 

 Were the vessel, charged with water so heated, 

 opened, it would be instantaneously emptied 

 by vaporization, since the whole caloric equi- 

 valent to its constitution as steam is present. 

 When, upon the other hand, steam is condensed 

 by contact with cold substances, so much heat 

 is set free as is capable of heating five and a 

 half times its weight of water, from 32 to 212 

 Fahrenheit. If the supply of heat to a copper 

 be uniform, five hours and a half will be re- 

 quired to drive off its water in steam, provided 

 one hour was taken in heating the water from 

 the freezing to the boiling point, under the at- 

 mospherical pressure. 



It thus appears that evaporation is a cooling 

 process, because the water is obliged to take 

 up an additional supply of heat to expand and 

 keep it in a state of invisible vapour. It is 

 equally plain, that when vapour is condensed 

 into mist, cloud, dew, rain, snow, and ice, it 

 must give out that extra supply of heat required 

 to convert it into vapour. Hence, in summer 

 and mild weather, evaporation is a cooling 

 process, whilst in winter the condensation of 

 vapour and congelation of water must tend to 

 prevent more excessive cold, by throwing out 

 latent heat into the atmosphere. The 1000 I 

 degrees of heat absorbed by watery vapour on 

 448 



EXUVLE. 



its expansion into vapour, must of necessity 

 be set free on its condensation and conversion 

 into water. 



EVERLASTING-PEA, BROAD-LEAVED 

 (Lathyrus latifolius'}. A perennial plant of the 

 vetch kind, which grows naturally in some 

 places ; is easily cultivated, and annually 

 yields a great burden of excellent provender, 

 and might be cultivated to advantage as a 

 green food for cattle, on any of the more strong 

 sorts of soil. See VETCH and VETCHLIXG. 



EVERLASTING, PEARLY (Gnaphalium 

 margaritaceum). One of the names of the 

 American cudweed. See CUDWKED. 



EVERY-YEAR'S-LAND. Such lands as 

 have been cropped with a brown and white 

 crop, or pulse and grain in alternation, for a 

 length of time, without any intervening fallow. 

 There are extensive common fields in Glou- 

 cestershire and other parts of England, which 

 have been conducted under this management 

 for perhaps centuries past. 



EXOTICS (Gr. farmer, foreign). In gar- 

 dening, a name given to plants which are not 

 natives, but have been introduced from some 

 other country. 



EXTRACTS (Fr. Extraits; Ger. Extracten). 

 The older apothecaries used this term to de- 

 signate the product of the evaporation of any 

 vegetable juice, infusion, or decoction ; whe- 

 ther the latter two were made with water, 

 alcohol, or ether; whence arose the distinc- 

 tion of aqueous, alcoholic, and ethereous ex- 

 tracts. 



Fourcroy made many researches upon these 

 preparations, and supposed that they had all a 

 common basis, which be called the extractive 

 principle. But Chevreul and other chemists 

 have since proved that this pretended principle 

 is a heterogeneous and very variable com- 

 pound. By the term extract, therefore, is now 

 meant merely the whole of the soluble matters 

 obtained from vegetables, reduced by careful 

 evaporation to either a pasty or solid consist- 

 ence. The watery extracts, which are those 

 most commonly made, are as various as the 

 vegetables which yield them ; some containing 

 chiefly sugar or gum in great abundance, and 

 are therefore innocent or inert; while others 

 contain very energetic impregnations. The 

 conduct of the evaporating heat is the capital 

 point in the preparation of extracts. They 

 should be always prepared, if possible, from 

 the juice of the fresh plant, by subjecting its 

 leaves or other succulent part, to the action of 

 a powerful screw or hydraulic press ; and the 

 evaporation should be effected by the warmth 

 f a water-bath, heated not beyond 100 or 120 

 Fahr. Steam heat may perhaps be applied 

 advantageously in some cases, where it is not 

 likely to decompose any of the principles of 

 the plant. But by far the best process for 

 making extracts is in vacuo, upon the princi- 

 ples involved in the process of evaporation. 



For exceedingly delicate purposes, the con- 

 centration may be performed in the cold, by 

 placing saucers filled with the expressed juice 

 over a basin containing sulphuric acid, putting 

 a glass receiver over them and exhausting 

 its air. 



EXUVLE (Lat). The cast-off parts or co- 



