-1859] WRITINGS OF JOSEPH HENRY. 273 



will be formed, and so on until all or nearly all the vapor 

 will be converted into water. In this way the steam power, 

 which has been accumulated from the heat of the sun, is ex- 

 pended in producing commotions of the atmosphere con- 

 nected with all the fitful — and many of the regular — meteoro- 

 logical phenomena of the globe. 



It may be objected to this part of the theory of Mr. Espy 

 that the condensation of the vapor in the atmosphere would 

 tend to contract it into a smaller space, consequently to 

 render it heavier, and thus neutralize the effect of the ex- 

 pansion due to the evolution of the latent heat. The effect 

 however from this cause is very small in comparison to that 

 due to the expansion of heat; and this will be plain when we 

 consider that the particles of vapor exist in the interstices 

 of the particles of air, and in a close vessel tend to in- 

 crease the volume only in proportion to their repulsive 

 force, which compared with that of the air, is small. For 

 example, if a quantity of dry air were inclosed in an India- 

 rubber bag, at a temperature of 60°, at the level of the sea, 

 its elastic pressure outward on the sides of the bag would be 

 equal to the weight of 30 inches of mercury, while the elas- 

 tic force of vapor would only be equal to half an inch of 

 mercury; so that we should have the enlargement of the bag 

 expressed by the last term of the following proportion : If 

 30 inches of mercury give one foot what will 30"5 give? 

 In this case, which is an extreme one, we see it would give 

 but a little more than 1 per cent., and hence the diminution 

 due to extracting the vapor from a quantity of air is very 

 small, and far less than the expansion due to the evolution 

 of heat. This will be evident from the following calculation 

 of the effect produced by the condensation of a pound of 

 vapor into water: It is known from direct experiment 

 that the condensation of one pound of vapor will raise 970 

 pounds of water 1°, or if it were possible to heat water thus high 

 it would raise one pound of water 970°; but the capacity of 

 air for heat is only one-fourth that of water; therefore the 

 condensation of one pound of steam would raise one pound 

 of air 3,880°, or 10 pounds of air 388°. The above calcula- 



18-2 



