Chap, xxxvii.j THE SPHEROIDAL STATE. 475 



Particles of a solid body, camphor for example, 

 may also pass into vapour without a previous tran- 

 sition to the liquid state. 



By a very simple apparatus, Wollaston showed 

 that water could be frozen by its own evaporation. 

 Two bulbs are connected by means of a tube. Water 

 is contained in one bulb, the other has an opening 

 communicating with the air. The water is boiled till 

 its steam fills the tube and the other bulb, and then 

 the communication of that bulb with the air is sealed. 

 The bulb which contains no water is then surrounded 

 by a freezing mixture which condenses the steam, 

 and thus tends to produce a vacuum. Vapour rises 

 from the water in the other bulb to fill the empty 

 bulb, and the rapid evaporation soon causes the water 

 to freeze. 



Increase of temperature favours evaporation. At 

 a given temperature the process will go on till the 

 atmosphere surrounding the liquid is saturated with 

 the vapour, in which condition the vapour is at maxi- 

 mum density, and exerts its maximum tension. If 

 the temperature be raised, the atmosphere is not 

 saturated for that temperature, and more vapour 

 will pass off till saturation is again produced. If 

 the temperature fall, the atmosphere is unable to 

 retain the former quantity of vapour, and some is 

 deposited in the liquid state. This explains the for- 

 mation of dew. 



The spheroidal state. If water be dropped on 

 a red-hot metal the water does not immediately hiss 

 and boil, as might be expected. It assumes a spherical 

 form, and rolls and tumbles on the hot metal. It can 

 be shown that the water is not in direct contact with 

 the metal, but is separated from it by an appreciable 

 interval. The reason is that by the intense heat of 

 the metal, steam is produced of sufficient tension to 

 support the liquid above the metallic surface. The 



