ON THE SOUnCJES AND EFPJECTS OF HEAT. 641 



degree: that of water is equal to this at the temperature 6i°, and is greater 

 or less nearly in proportion to the distance from 39°, where it hegins, hut 

 in high temperatures it varies less, since it is not quite four times as great at 

 the heat of boiling water. The expansion of spirit of wine at 70° is six times 

 as great as that of mercury: its utmost variation is much less than that of 

 water, although it is at least twice as great in some parts of the scale as in 

 others. 



It has already been observed that an elevation of temperature consider- 

 ably diminishes the powers of cohesion and of repulsion in solid bodies: the 

 same is also true of liquids; for the height to which a liquid rises in a capil- 

 lary tube is diminished somewhat less than -^o- for each degree of Fahren- 

 heit that the temperature is raised, the force of superficial cohesion being 

 diminished both by the diminution of the immediate actions of the particles, 

 and by that of the distances to which they extend. 



When the temperature of a liquid is so much elevated as to become equal 

 to that of its vapour in a state capable of sustaining the atmospherical pres- 

 sure, or any other pressure which may be substituted for it, a certain por- 

 tion of the liquid is converted into vapour, and the heat being generally 

 applied at the bottom of the vessel, the vapour rises up in bubbles, and the 

 effect is called boiling. The whole liquid is not converted at once into 

 vapour, because a certain quantity of heat appears to be consumed by the 

 change, and a constant supply of heat is necessary, in order that the opera- 

 tion may be continued. 



It is not, however, only at the boiling point that a fluid begins to be con- 

 verted into vapour: the pressure of the atmosphere is not sufficient wholly to 

 prevent the detachment of a certain quantity of vapour from its surface, at 

 a temperature which is incapable of supporting it separately in the form of 

 Steam in the open air, and it may be thus suspended, when mixed either with 

 common air, or with any other elastic fluid, at the ordinary temperature of the 

 atmosphere. And it appears that thet^uantity, which is thus suspended, bears 

 very nearly some constant proportion to the density of which the steam is 

 capable at the given temperature in a separate state, the interposition of the 

 air either not affecting the distance at which the cohesion would take place, 



