12 SCIENCE IN SHOUT CHAPTERS. 



heat upon materials similar to those of which our earth is 

 composed? 



Let us first take the case of water, which, for reasons I 

 have stated, should be regarded as atmospheric, or univer- 

 sally diffused matter. 



This brings us to a subject of the highest and widest 

 philosophical and practical importance. I refer to the an- 

 tagonism between the force of heat and that of chemical 

 combination, to which the French chemists have given 

 the name "dissociation." Having myself been unable to 

 find any satisfactory English account of this subject at a 

 time when it had already been well treated by French and 

 German authors, in the form of published lectures and 

 cyclopasdia articles, I assume that others may have en- 

 countered a similar difficulty, and therefore dwell rather 

 more fully upon this part of my present summary. 



It appears that all chemical compounds may be decom- 

 posed by heat, and that, at a given pressure, there is a def- 

 inite and special temperature at which the decomposition 

 of each compound is effected. For the absolute and final 

 establishment of the universality of this law further investi- 

 gations are necessary, actual investigations having estab- 

 lished it as far as they have gone, but these have not been 

 exhaustive. 



There appears to be a remarkable analogy between dis- 

 sociation and evaporation. When a liquid is vaporized, a 

 certain amount of heat is "rendered latent/' and this quan- 

 tity varies with the liquid and with the pressure, but is 

 definite and invariable for each liquid at a given pressure. 

 In like manner, when a compound is dissociated, a certain 

 amount of heat is " rendered latent," or converted into dis- 

 sociating force, and this varies with each compound and 

 with the pressure, but is definite and invariable for each 

 compound at a given pressure. Further, when condensa- 

 tion occurs, an amount of heat is evolved, as temperature, 

 exactly equal to that which was rendered latent in the evap- 

 oration of the same substance under the same pressure; and, 

 in like manner, when chemical re-combination of dissociated 

 elements occurs, an amount of heat is evolved, as tempera- 

 ture, exactly equal to that which disappeared when the 



