Spheroidal Condition of liquids is the name usually given to a series of very singular phenomena discovered by Leidenfrost (1715-94), but first carefully investigated by Boutigny (1858). Indeed one, at least, of those 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. The drop will either glance off the surface without wetting it or will spread over the surface and evaporate, according as the iron is or is not too hot.
The common experimental method of exhibiting the spheroidal condition is easily performed thus: A metallic disc, slightly concave, like a watch-glass, is heated by a lamp, and water is cautiously dropped on it from a pipette. If this be done before the disc is sufficiently heated the water boils almost explosively, and is dispersed at once in
vapour. But when the disc is hot enough the water remains suspended, as shown in the cut, above the surface; and the drop,
when small, takes nearly the form of an oblate spheroid. The most curious fact connected with the experiment is that the water does not boil. In fact it evaporates so freely that the heat carried off from it, as latent heat, by the vapour which is constantly formed keeps its temperature somewhere about 206° F. only. Boiling water, dropped on a red-hot plate of metal, instantly assumes the spheroidal state, and is cooled six degrees below boiling. 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 disc being dependent on the boiling-point of the substance. A good example of a solid entering this state is furnished by dropping crystals of iodine on a hot platinum disc. It is not necessary that a metal plate be used. For example, a watch-glass will suffice for the experiment; but hot water must be dropped on it, else the glass will crack. Also we may obtain ether, and even water, in the spheroidal state over the surface of hot oil. The explanation of the phenomenon is to be sought for in the existence of a cushion of vapour between the hot plate and the drop. The high temperature of the plate causes a rapid evaporation of the lower surface of the drop. The vapour particles coming into contact with the hot plate are raised in temperature, that is, have their kinetic energy increased. They tend to rebound with increased energy back upon the drop, whose weight they are thus able to support.
An interesting illustration of the spheroidal state is the fact that the hand may be dipped for a short time with impunity into melted lead, and even into melted copper. The vapour 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.