SPHEROID′AL STATE. It is found that water, or any other volatile liquid, thrown on a metallic plate heated to dull redness, is not resolved into vapour, but, assuming a somewhat globular form, remains intact, until the temperature becomes sufficiently lowered to allow of contact between the liquid and the heated surface. It is then immediately volatilised. M. Boutigny, who fully investigated this subject, has also shown that the same thing happens when a solid body containing water is substituted for the liquid in the above and similar experiments. Thus, the finger or hand, under certain restrictions, may be thrust, with perfect impunity, into a stream of molten metal, and ice may be produced by throwing water into a red-hot crucible. This last experiment, as performed by MM. Boutigny and Prevostaye, is essentially as follows:—A thick platinum crucible, of the capacity of 1 fl. oz., is heated to redness over a powerful spirit lamp, and some liquid anhydrous sulphurous acid (a very volatile substance) poured into it by means of a pipette; the acid assumes a spheroidal form, and does not evaporate; a few drops of water are now introduced into the sulphurous acid in the same way; the diluted and slightly cooled acid instantly flashes off in vapour, and, robbing the water of its caloric, leaves the latter in a frozen state; and, if the operator seizes the right moment, a solid lump of ice may be thrown out of the red-hot crucible.
By substituting for anhydrous sulphurous acid a mixture of solid carbonic anhydride and ether, and for water a few grains of quicksilver, this latter may be reduced to the solid condition, and may be turned out of the red-hot crucible in the form of a small frozen mass.
The spheroidal condition of “liquids is a complicated result of at least four distinct causes. Of these the most influential is the repulsive force which heat exerts between objects which are closely approximated towards each other. When the temperature reaches a certain point actual repulsion between the particles ensues.
“Besides this repulsive action occasioned by heat, the other causes which may be mentioned as tending to produce the assumption of the spheroidal condition by the liquid are these:—
“1. The temperature of the plate is so high that it immediately converts any liquid that touches it into vapour, upon which the spheroid rests as on a cushion.
“2. This vapour is a bad conductor of heat, and prevents the rapid conduction of heat from the metal to the globule.
“3. The evaporation from the entire surface of the liquid carries off the heat as it arrives, and assists in keeping the temperature below the points of ebullition. The drop assumes the spheroidal form as a necessary consequence of the action of cohesion among the particles of the liquid, and the simultaneous action of gravity on the mass.”[182]
Boutigny found that, when a liquid in a state of ebullition was brought into contact with a surface heated to such a degree as to cause the liquid to assume the spheroidal state, its temperature immediately fell 3° or 4° C. below the boiling point.
All liquids are capable of assuming the spheroidal condition; but, as the temperature necessary for this purpose, varies with the boiling point of each liquid (the lower the boiling point the lower the temperature necessary, and vice versâ), it follows that the conducting surface requires to be differently heated for each liquid. The exact temperature to which the plate should be heated to produce the spheroidal condition in any liquid, depends partly upon the conducting power of the plate, and partly upon the latent heat of the vapour; the less this is, the more nearly the temperature of the plate approximates to the boiling point of the liquid.
Boutigny believed that the temperature of each liquid, when in the spheroid condition, was as invariable as that of its boiling point; but Boutan has demonstrated that this is a not quite accurate statement, since the temperature of the same liquids, when assuming the spheroidal form, is liable to slight divergence.