EBULLI′′TION

The state of boiling, or the agitation of a liquid arising from its rapid conversion into vapour by heat. Ebullition occurs in different liquids at very different temperatures, such temperatures being called their ‘boiling-points.’ Under the same circumstances the boiling-points are constant, and by observing them the chemist is often able to distinguish liquids which much resemble each other. The boiling-point of the same liquid may, however, vary considerably under different circumstances. The causes which induce variation are increased or diminished atmospheric pre ure, the greater or le depth of the liquid, and the character of the containing ve el. Thus boiling water is colder by some degrees when the barometer is low, in bad weather, or at the top of a hill, than when the barometer is higher, in fine weather, or at the bottom of a valley or mine. There is a very simple and beautiful experiment, illustrative of the effect of diminished pre ure in lowering the boiling-point of a liquid. A little water is made to boil for a few minutes in a flask or retort placed over a lamp, until the air has been expelled, and the steam i ues freely from the neck. A tightly fitting cork is then inserted, and the lamp at the same moment withdrawn. When the ebullition ceases, it may be renewed at pleasure for a considerable time by the affusion of cold water, which, by condensing the vapour within, occasions a partial vacuum. Liquids in general boil from 60° to 140° lower than their ordinary boiling-points when heated in vacuo . The following table furnishes very exact information respecting the effect of increasing pre ure upon the boiling-point of water:— Boiling water contained in a deep ve el is hotter than that in a shallow one, on account of the greater resistance in the one case than the other to the escape of the steam. It is also found that fluids boil at a lower temperature and more quietly in ve els with rough and spicular surfaces, than in those with smooth or polished ones. The boiling-point of water, as marked on the scale of the thermometer, is 212° Fahr., but in gla ve els, under common circumstances, it varies from 212·254° to 215·6°; whilst in perfectly pure and smooth gla ve els water may be heated to 221° Fahr. without boiling. That the elevation of the boiling-point in this case is due to the nature of the surface, may be at once demonstrated by throwing into water, about to boil in a gla matra , a little iron filings or coarsely powdered gla , when ebullition will commence with almost explosive violence, at the same time that the temperature of the fluid will sink about 2° Fahr. Table I. — Boiling-points of Water at different Pre ures. By Mr C. Greville Williams . | Boiling-point ° Fahr. | Barometer Inches. | Boiling-point ° Fahr. | Barometer Inches. | | 184 | 16·676 | 200 | 23·454 | | 185 | 17·047 | 201 | 23·937 | | 186 | 17·421 | 202 | 24·441 | | 187 | 17·803 | 203 | 25·014 | | 188 | 18·196 | 204 | 25·468 | | 189 | 18·593 | 205 | 25·992 | | 190 | 18·992 | 206 | 26·529 | | 191 | 19·407 | 207 | 27·068 | | 192 | 19·822 | 208 | 27·614 | | 193 | 20·254 | 209 | 28·183 | | 194 | 20·687 | 210 | 28·744 | | 195 | 21·124 | 211 | 29·331 | | 196 | 21·576 | 212 | 29·922 | | 197 | 22·030 | 213 | 30·516 | | 198 | 22·498 | 214 | 31·120 | | 199 | 22·965 | 215 | 31·730 | The boiling-point of water contained in ordinary ve els may be raised considerably above 212° Fahr., by the addition of saline matter, as will be seen in the following table, extracted from Mr C. G. Williams’s excellent ‘Handbook of Chemical Manipulation,’— Table II. — Boiling-points of Saturated Solutions of various Salts at the ordinary Atmospheric Pre ure. By C. G. Williams . | Name of Salt. | Boiling-point. | | Chloride of calcium | 355° | Fahr. | | Acetate of soda | 256 | ” | | Nitrate of soda | 246 | ” | | Sal-ammoniac | 236 | ” | | Common salt | 224 | ” | | Cream of tartar | 214 | ” | The above solutions are suitable for chemical baths. With the exception of the first, they furnish in their boiling-points temperatures, as nearly as can be obtained, 10° above each other. They were chosen by Mr Williams because, in ‘fractionating’ volatile substances, it is usual to separate the distilled products by differences of temperature equal to 10° Fahr. In long operations it is found inconvenient to employ a saturated saline solution for a bath (by which the highest temperature would be obtained), as the constant evaporation of the water induces the crystallisation of the salt. It is hence usual to keep it considerably below that point. The following table, compiled chiefly from the pages of Dr Miller’s ‘Elements of Chemistry,’ gives the boiling-points of several interesting substances. Table III. — Boiling-points of various Liquids at the ordinary Atmospheric Pre ure. | Name of Substance. | Boiling-point. | | Liquid carbonic acid | -108° | Fahr. | | Liquid sulphurous acid | + 17·6 | ” | | Chloric ether | 51·9 | ” | | Aldehyd | 69·4 | ” | | Ether | 94·8 | ” | | Bisulphide of carbon | 118·5 | ” | | Bromide | 145·4 | ” | | Wood spirit | 149·9 | ” | | Alcohol (sp. gr. ·815) | 173·1 | ” | | Benzol | 176·8 | ” | | Dutch liquid | 184·7 | ” | | Acetic acid | 243·1 | ” | | Sulphur (melted) | 609· | ” | | Mercury | 662· | ” |