SPHEROIDAL CONDITION of liquids is the name usually given to a series of very singular phenomena discovered by Leidenfrost, but first carefully investigated by Bou tigny. Indeed, one, at least, of these phenomena has been popularly known for a very long time, being the foundation of the rough practical method of determining whether or not a flat-iron is so hot as to be likely to singe the linen to which it is to be applied. The test consists simply in letting a drop of water fall upon the iron; if it be not too hot, the drop spreads over the surface and evaporates. If it be too hot, the drop at once glances off the iron without wetting it.
The common experimental method of exhibiting the spheroidal condition is easily performed thus: A. metallic disk, slightly concave, like a watch-case, is heated by a lamp, and water is cautiously dropped on it from a pipette. If this be done before the disk is sufficiently heated, the water boils almost explosively, and is dispersed at once in vapor. But when the disk is hot enough, the water remains suspended above the sur face; and the drop, when small, takes nearly the form of an oblate spheroid. Various proofs have been given, though they are obviously unnecessary. that there is no contact in this case. Thus, if the disk be very nearly flat, light passes freely between it and the drop. Again, if one pole of a galvanic battery be connected with the disk, and the other be dipped into the drop, a galvanometer interposed in the circuit shows that no current passes. By heating the disk sufficiently, and dropping on the water very carefully, we may easily keep in the spheroidal state as much water as, if not more than, it could hold when cold. The explanation of the phenomenon is not yet quite clear; but there is no doubt that the radiant heat from the disk raises vapor so freely from the surface of the drop nearest it, as to interpose a cushion of dense and highly heated vapor between them, on which the drop,as it were, floats; the pressure of the vapor balancing its weight. This is not, however, a quite complete explanation of the experiment, and it would require too much detail to examine it more closely. But the most curious fact connected with the experi ment is, that the water does not boil. In fact, it evaporates so freely that the heat car ried off from it, as latent heat, by the vapor which is constantly formed, keeps its tem perature somewhere about 206° F. only. This suggests a curious experiment, which is found to succeed. Boiling water, dropped on a red-hot plate of metal, instantly assumes the spheroidal state, and is cooled six degrees below boiling.
It is not necessary that a metal plate be used—a wateh-glass will suffice for the experiment; but hot water must be dropped on it, else the glass will crack.
Other liquids, and even some bodies which are solid at ordinary temperatures, can be easily brought into the spheroidal state—the lowest requisite temperature of the disk being dependent on the boiling point of the substance. Thus, while water has a tem
perature of 206° F. in the spheroidal state, the disk must have a temperature of 340° F. at least. For alcohol, these temperatures are 168° F. and 270° F.; for ether, 94° F. and 140° F. A good example of a solid entering this state is furnished by dropping crystals of iodine on a hot platinum disk.
It is not necessary that the disk should be solid; it is easy to obtain ether, and even water, in the spheroidal state over the surface of hot oil—but great care is required, as explosions are apt to occur, in which case the hot oil is freely thrown about.
Many cases of bursting of steam-boilers, otherwise apparently inexplicable, seem to be attributable to this condition of matter. Thus, if we suppose that the water-supply has run low, and the boiler has been overheated, it is conceivable that the contents may sometimes be in the spheroidal state. The addition of cold water in such a case would bring them suddenly in contact with the overheated metal, and large quantities of steam would be generated with violence.
A very singular experiment, the freezing of water on a red-hot plate, is easily per formed by the help of this property of matter. Liquid sulphurous acid is so volatile as to have a temperature of 13° Fahr. only, when in the spheroidal condition. As this is 19° under the freezing-point of water, if a little water be dropped into the spheroid of acid, it is at once frozen, and the pellet of ice can be dropped on the hand from the still red hot plate.
Even mercury can be frozen by a similar process, but as much greater cold is required, the substance in the spheroidal state is a mixture of solid carbonic acid and The hand may be dipped for a short time with impunity into melted lead, and even into melted copper. The vapor, instantly raised, from the moisture of the skin, prevents, so long as that moisture lasts, more than an endurable amount of radiant heat from reaching the hand, and also prevents direct contact. It is probable that a knowledge of some forms of this phenomenon, in old days, was employed by priesteraft for the pur pose of protecting, when it was desirable to do so, the victims of the ordeal (q.v.) by fire.
The phenomenon may easily be reversed. Thus, a red-hot silver ball, dropped into a a vessel of water, is seen to glow for some time, till it has so far cooled that the comes into contact with it, when we have, as in the other form of the experiment, an immediate and violent formation of vapor. The success of this experiment is greatly aided by the addition of some strong ammonia to the water.