430 On the Chemical Phenomena of Heat. [Juny, 
atoms ; this is too evident to require demonstration. The difference 
between a chemical union and mechanical mixture is this; the 
particles of which each species of matter concerned is composed 
attract those of the other rather than its own .*. two or more dis- 
similar particles form a system, the union between them being more 
intense than that force which binds together the aggregate. Sup- 
pose A, B, C, (Fig. 7) to be three dissimilar particles in chemical 
combination, B and C are in = librio, i.e. the forces which act 
upon them balance each other; if they have freedom of motion, 
they may be made to move round their centre of gravity; let them 
combine with a third particle, A; the whole must now be in = 
librio; and on placing together any number of such systems, their 
centres of gravity H, K, will be in = librio. 
Caloric acts in two ways upon compounds: it tends to separate 
the centres of gravity of the systems, and also destroy the union 
between the individual particles of which these systems are com- 
posed; for since the forces act upon all the particles, an increase of 
heat must increase their distance from each other. Experiments 
which prove the fact are numerous: alcohol boils at a moderate 
temperature, distilling without alteration : expose it to a red heat by 
passing it through an ignited tube, and it is decomposed. Calorie 
acts in several apparently opposite ways upon compounds. In some 
cases it promotes, and in others destroys chemical union. When it 
promotes chemical union, the attraction of the heterogeneous par- 
ticles is not sufficient to overcome their attraction for each other ; 
at the ordinary temperature of the atmosphere, an elevation of tem- 
perature sometimes lessens one and sometimes the other of these 
attractions, according to the altitude and density of the calorific 
atmospheres, so as at one time to cause bodies to combine, at an- 
other to destroy their union. For example, the particles of copper 
have for each other so strong an attraction that their attraction for 
oxygen, and that of sulphuric acid for the oxide of copper, cannot 
dissolve or in any way act upon the copper in the cold; when the 
acid is elevated to its boiling point, it is partly decomposed, the 
particles of the copper are oxygenated, and the oxide dissolved ; 
when the substance thus formed is ignited, it is again decomposed, 
the acid is expelled, and a part of the copper seems to be reduced to 
its metallic state. From this it is manifest that the order of affinity 
must always depend in part upon temperature ; and the affinities, or 
rather attractions of different bodies for each other, evidently depend 
primarily upon the quantities required for saturation; yet tempera- 
ture, insolubility, elasticity, specific gravity, and electricity, have 
so much influence that the real law is with difficulty ascertained. 
(Zo be continued.) 
