Spheroidal State

It is well known that when a drop of water is allowed to fall on a red-hot sheet of metal, such as platinum, it does not touch the metal, nor does it boil although rapid evaporation takes place, but it assumes the spheroidal state, moving about over the hot surface like a globule of mercury on a table. Careful experiments1 have shown that the temperature of the water under these conditions does not reach 100°, and the explanation of this fact is probably that the aqueous vapour surrounding the spheroidal drop is diluted with air, so that its partial pressure is less than the total atmospheric pressure. There are two reasons, either of which would be sufficient, why ebullition cannot take place :1. Heat is received from without towards the surface, from the whole of which evaporation can take place freely, as there is no contact between the water and the heated platinum ;

2. The vapour pressure corresponding to the temperature of the water is lower than the atmospheric pressure, so that a bubble, if formed, would at once collapse.

Wet And Dry Bulb Hygrometer

Again, the difference in temperature which is usually observed between the wet and dry bulb in the ordinary hygrometer depends on the difference between the partial pressure of the aqueous vapour in the air and the maximum pressure possible at the temperature of the dry bulb, that is to say, the vapour pressure of water at that temperature.

Non-Miscible Liquids

Lastly, as will be shown later on, the fact that when two non-miscible liquids are distilled together, the boiling point is lower than that of either component when distilled alone, may similarly be explained by the law of partial pressures, the vapour of each liquid acting like an indifferent gas towards the other.

1 Balfour Stewart's Treatise on Heat (6th edition), 129.