148 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



changes occurring in the interior of cells. The constituents of all 

 chemical compounds are held together by their chemical affinities, and 

 compound atoms hence exhibit a lesser tendency to form new combina- 

 tions than do free, uncombined atoms. When, however, a combination 

 is broken up, as by some external force, the separated atoms again tend 

 to unite. There is again a difference in the stability of chemical com- 

 pounds. In other words, new combinations are more readily formed in 

 some instances than in others. Thus, the loosely -held oxygen atom in 

 hydrogen peroxide (HjOj) is much more readily given up to form fresh 

 combinations than the closely-held atom of oxygen in H 2 0. Thus, forces 

 which only exist as a tendency to produce motion are termed potential 

 forces; those, on the other hand, which actually manifest themselves in 

 movements are actual or kinetic forces. The tendenc}' to combine is, 

 therefore, a potential force, which varies according to the free or com- 

 bined state of the atoms, and, in the latter case, according as the atoms 

 are held in loose or close combination. The atom released from chemical 

 combination acquires an increased potential force, while an atom which 

 passes from the free to the combined state loses in potential. In 

 ever}- chemical decomposition, or the passage from close to loose com- 

 pounds, the potential force is developed, while in the formation of other 

 chemical compounds, or the passage from loose to close compounds, the 

 potential is diminished. The separation and union of the elements and 

 formation of chemical compounds are, therefore, movements of the 

 elements ; consequently, in their decomposition or composition actual 

 forces are produced. When atoms unite, a part of the force which before 

 existed as a tendency to form combination (the potential force) is 

 converted into actual or kinetic force, and the combination has lost 

 potential to that extent to which the tendency to form combinations is 

 satisfied. In decomposition the exact opposite holds. In order to break 

 up combinations an external force is required, and the amount of poten- 

 tial acquired by the atoms is precisely equal to the actual force employed. 

 It follows from this that in the act of every chemical composition actual 

 force is liberated, and in every chemical decomposition actual force is 

 rendered latent, or is converted into potential. In the first case the force 

 developed is equal to the potential lost in combination, and in the second 

 case the actual force rendered latent equals the potential force developed. 

 Where a loss of potential occurs, it is compensated for by a proportional 

 development of actual force, and vice versa. 



The same rule applies to all forces in nature. Every development 

 of force is to be regarded as a change from actual to potential force or the 

 reverse, or the transformations of different forms of actual force. This 

 law is known as the conservation of energy. The forms of actual force 

 that we are familiar with are movements of masses, light, heat, and elec- 



