242 ANNUAL OF SCIENTIFIC DISCOVERY. 



nomena considered under this name, and to the speculation, that a 

 new law superseding the ordinary laws of heat was here illustrated. 

 The object of the communication was principally to show, that these 

 phenomena are not at all rare, and that they require no new law for 

 their explanation. In the experiment of dropping water, upon a hot 

 polished metallic surface, we have three bodies concerned in the phe- 

 nomena that follow ; viz. the supporting surface, the water, and the 

 layer of vapor interposed between. The water rests upon a cushion 

 of steam continuously evolved by heat from the water, and assumes 

 ^rounded margins as the result of the gravity of its particles towards its 

 own centre. Its condition approximates to that of a liquid entirely 

 surrounded by a uniformly yielding elastic medium, as a drop of mol- 

 ten lead in air, for example, and permits a proportional approximation 

 to the spheroidal form. It is essential only that there be two bodies, 

 one of which must be fluid, and between which affinity is wanting; 

 elevated temperature is not necessary. Corresponding with water is 

 the ordinary experiment known by the name of Leidenfrost ; ether, 

 alcohol, a variety of essential oils, etc., may be. employed. The. con- 

 ditions are the same ; the liquid evolves a vapor, which, constantly is- 

 suing, prevents contact with the supporting surface. Here are three 

 bodies and a high temperature. Ether and oil dropped on water as- 

 sume the spheroidal state. They have no affinity for the water. Here 

 but two bodies and no heat are required. Quicksilver upon glass, and 

 also water upon glass covered with fine dust, take on the spheroidal 

 state. Potassium thrown upon water assumes this state. In the 

 decomposition of the water producing potassa, hydrogen is evolved, 

 and with the heat arising from this union and that of the potassa with 

 water, forming hydrate of potassa, water is vaporized, which with the 

 hydrogen keep the floating sphere and water apart. The dew-drop 

 presents the spheroidal, but little differing from the spherical, state. It 

 rests in most cases upon the hair or down with which leaves are cov- 

 ered, and is not in contact with the leaves or twigs. The bead, as it 

 is called, which appears at the surface of some liquids when violently 

 shaken, and of which we have a familiar example on the water around 

 the prow of an advancing sail-boat, is an instance of the spheroidal 

 state of great interest ; what prevents the prompt reunion of the bead 

 with the mass is not so obvious. 



There is a fact in the history of the barometer, noticed by Prof. 

 Guyot, which may have some influence in explaining this phenomenon. 

 When pure mercury is introduced into a barometer tube, and after- 

 wards boiled, so as to expel all air, upon erecting the tube in the cis- 

 tern, the top of the column presents a certain curve, the meniscus, 

 the character of which is dependent on the composition of the glass 

 and the diameter of the tube. If now a bubble of air be introduced 

 and then carefully removed, on erecting the barometer again, the mer- 

 cury will stand at the same elevation as before, but the meniscus will 

 be less convex. Here, as in the case of the bead, it may be con- 

 ceived that the mercury and water have condensed an infinitely thin 

 layer of air upon their surfaces, which provides for the phenomena of 

 the spheroidal state in the latter instance, and which modifies the affin- 

 ity of the mercury for the glass in the former. 



