Page:The American Cyclopædia (1879) Volume II.djvu/815

 BOILING POINT 795 sel. If a basin is placed about the orifice to catch the falling liquid, which in the presence of the expanding vapor has parted with much of its heat, and convey it back again to the cylinder, a period of comparative quiet will follow. During this time the temperature of the column will increase, and bubbles of steam will rise higher and higher, until at last, when they have attained sufficient force, the violent expulsion of steam and water will be repeated. The geysers in Iceland, and the great Ameri- can geysers at the head waters of the Missouri river, are examples in nature of the boiling of water in vertical tubes. There are some cir- cumstances attending the boiling of water be- sides external pressure which must he taken into consideration in making experiments, or correct results will not be reached. If water is boiled in a well cleaned glass flask which is perfectly smooth inside, it will, when the barom- eter stands at 29'922 inches, reach a tempera- ture of 214. If the flask had been rinsed with a solution of potash, the boiling might not have occurred below 215 or 216. The reason as- signed for these phenomena is that the perfect cleaning of the glass in one case, and the pres- ence of a small quantity of potash in the other, increases the cohesion of the water and glass to such a degree as to demand an increase of heat to effect a separation between them. If water be boiled for a long time in a flask, and not in a vessel where the surface is freely ex- posed to the air, it will be observed, especially if the heat is moderately applied to the centre of the bottom, that the ebullition becomes more or less irregular or jerking. If the water is allowed to cease boiling for a few moments, and the heat is carefully applied, the tempera- ture may be raised as high as 220 before any bubbles of steam will be formed, when the boil- ing will take place with a sudden leap, accom- panied by a rapid decrease of temperature ; then there will be another period of quietude, succeeded by another violent evolution of va- por. These effects are heightened, if instead of using an open flask the water is boiled in a partial vacuum of its own vapor. This may be done by removing the lamp and corking the neck of the flask after the air has been as far as possible expelled. If we now turn cold water over the flask, the vapor within will be partially condensed, and the boiling will recommence and will continue even if the flask be plunged into cold water, until its temperature is reduced much below blood heat, and indeed as long as the tension of the vapor above the water can be kept below the tension of the vapor which the water is capable of yielding. Near the con- clusion the ebullition becomes very irregular and jerking ; and if the flask is placed in a re- tort stand and gently heated at the bottom, the bursts of vapor will be more explosive than during the cooling process, and sometimes the flask will be thrown from the stand. The ex- planation which is generally received is this : Water in its natural state contains a consider- able quantity of atmospheric air. Boiling ex- pels a portion, but not all of it, unless it has con- tinued a long time. While this expulsion of air is taking place, if only in exceedingly small quantities, little bubbles of it are formed into which the steam can enter and expand ; but when the air is all expelled, the molecules of water will not separate from each other as readily as they passed into the air chambers. It seems as if there needed to be an opening or a point of diminished pressure somewhere in order that the particles of water at 212 F. may expand into vapor. Dufour has very care- fully studied this subject. In experimenting with water he used a mixture of oil of cloves and linseed oil, which had been previously heated to 390 F. and allowed to cool. The water, heated to 170, was carefully dropped in so as not to disturb the film of oil which coated the bottom of the vessel, and the temper- ature was gradually raised. The boiling point would invariably be passed and a heat of 230" or 236 reached before any manifestation of ebullition could take place. Then an explosion would occur and the remainder of the globule of water would be violently driven to one side. He succeeded in raising some small globules to 347 F., a temperature which would cause wa- ter with an exposed surface to boil under a pressure of more than eight atmospheres. The passage of sparks from a Leyden jar would produce violent explosions ; so also would a weak galvanic current, but in a less degree. In the latter case Dufour attributed the eft'ect to the production of bubbles of gas at the ends of the conducting wires. He also found that when the surface of water was covered with a thin film of oil its temperature could be raised considerably above the boiling point. The in- vestigations of Prof. Donny of Ghent, who has succeeded in raising water far above its boiling point when not enclosed in oil or other sub- stances, have added much to the stock of knowledge on the subject. Prof. Kopp and others have extended researches to various other liquids, and have found that many of them also possess the property of being raised under certain circumstances several degrees above their boiling points. Tims, methylic alcohol, whose boiling point is 141-8 F., may be raised by changing the nature of the vessel to 152. In estimating the boiling point of a liquid Dufour very sensibly suggests that we should take the lowest temperature at which a liquid can be made to boil under the proper conditions. That an examination of this sub- ject in relation to the cause of steam-boiler ex- plosions would lead to important improvements is most probable. That the temperature of the water in the boiler of a steam engine may be raised considerably above the boiling point is very possible, as for instance when the engine has been standing quiet for some time, and the water has been deprived of most of its air. Under such circumstances a disturbance of rest would cause an explosive burst of vapor, pro-