BOILING OF LIQUIDS (ante). As will be understood from the above, the terms liquid and boiling-point are entirely relative, depending upon external agents and upon each other. The statement that water is a liquid is only true under certain conditions. In the arctic regions it is a solid, and in a vessel heated to under ordinary atmos pheric pressure it is a vapor or gas. Nitrous oxide is a liquid under ordinary atmos pheric pressure when reduced below 126° below zero, and the same is true of carbonic acid when reduced to 10t3.76'. Pressure, however, is capable of reducing both these gases to liquids, and modern experiments with various substances are now common in which carbonic add is liquefied by pressure. Ammonia, commonly a bas, is a liquid when reduced to —28.66°. This substance is capable of being absorbed by a very small volume of water under heavy pressure, or. at least, of occupying a very small volume; for we cannot. say that the gas is really absorbed; the water assists the pressure in bold ing the gas in a liquid form. Advantage is taken of this in the working of a certain class of ice-making machines, called ammonia machines (q.v.). Some of the machines, however, depend upon the vaporization of ammonia, anhydrous. or nearly so, for the absorption of sensible heat. The following is a table of the boiling-points of various substances: The investigations of prof. Kopp indicate certain remarkable laws connecting- the boiling-points of classes of liquids with their chemical constitution. The following tables. calculated from the observations of prof. Kopp and others, show that in the group of alcohols, and the acids derived from them by oxidation—both of winch differ in con stitution by one molecule of CIL—there is a difference of very nearly 34.2° F. between successive members of the series; and that, moreover, the difference iu the boiling-points between the alcohols and their respectively derived acids is about 72°.
Other analogous correspondences in the boiling-points of liquids and their chemical constitution were observed; thus in the series of hydrocarbons, homologous with ben zoic, C,I12, a difference in the series of was attended with a difference of boiling point of about 43°.
The molecular constitution, or. more strictly speaking, the mutual relations between the molecules of liquids. particularly as regards water. whose affinities are so numerous. exerts a great influence not only upon the boiling-poi'.it. but upon the nature or manner of ebullition. Thus, if a clean glass flask is partially filled with ordinary, and, of course, more or less aVrated spring water, and heated rapidly with a spirit-lamp, nearly all the air will be expelled first, but before all the air is thus expelled ebullition will Commence, and at it point very slightly below 212°. After a little time. more of the air having dis appeared, but not entirely, the boiling-point (at 30 in. mercurial pressure) will be 212°. By continuing the boiling, however, the tuOde of ebutlitiork will be found to have changed. If the flask is held quite still there will be intervals of time—althoug,h the application of heat is constant—when ebullition will cease; and these intervals the temperature will rise. If the heat is taken away for a few moments so as to allow the water to come to a state of comparative rest, and then reapplied, ihe temp(irature may be raised to 220° before ebullition commences, when it will be decidedly explosive. If now the flask is corked tight, and a partial vacuum formed in the space occupied by vapor, boiling will go on until the water is quite cool, but the boiling will he of the explosive character observed in the later periods of application of heat, and when quite cool will be more irregular, partly in consequence of the reduction of atmospheric pres sure, but more particularly, probably because of the increased cohesion between the contiguous molecules of water by reduction of heat.