-1859] WRITINGS OP JOSEPH HENRY. 227 



and that this also is an efficient means by which colder por- 

 tions of the earth are mollified in temperature. 



In the explanations we have thus far given we have spoken 

 of the increase of the repulsion of the atoms of water by an 

 increase of heat. By this we mean the increased tendency 

 which they have to separate from each other with a force 

 which resembles simple repulsion, but which, if we adopt 

 the vibratory theory of heat, will be due to the increased 

 intensity of the oscillation of the particles. We have also 

 employed the usual term "latent heat" to express the heat 

 which disappears when a solid is converted into a liquid, or 

 a liquid into a vapor — though this, according to the new 

 theory of heat, would be expressed by the quantity of vibra- 

 tion or mechanical energy which is absorbed in the change 

 of state of the body and which will re-appear when the re- 

 verse process takes place. To illustrate this suppose an up- 

 ward impulse be given to a ball sufficient to throw it upon 

 a shelf. In this case we may consider the mechanical energy 

 as having been expended in producing this effect, although 

 it is ready again to make its appearance and to do work when 

 the ball is suffered to fall again to the level whence it was 

 projected. 



Vapor in air. — We are also indebted to Dr. Dalton for 

 another important series of experiments which relate to the 

 mingling of air and vapor. In the experiments before given 

 the vapor was weighed, and its temperature and tension 

 determined in a separate state and unmingled with the air. 

 To ascertain the effect which would be produced on the ten- 

 sion of vapor when suffered to be exerted in a space alreadj'' 

 occupied with air of different densities. Dr. Dalton employed 

 the same method of experimenting previously described. 

 A barometer tube was filled and inverted, as before, in a 

 basin of mercury, a quantity of air was then admitted, which 

 rising into the Torricellian vacuum, pressed by its elasticity 

 on the surface of the mercury and caused it to descend a 

 given number of divisions of the scale which were accu- 

 rately noted; a small quantity of water was next admitted, 

 which rising to the top of the mercurial column was after a. 



