INTRODUCTION. xli 



changed into heat. Chemical affinity is a form of potential energy obtaining 

 between the most different atoms, which during chemical processes is changed into 

 heat. Conversely, in those chemical processes where strong affinities are dissolved, 

 and chemically-united atoms thereby pulled asunder, there must be a diminution of 

 temperature, or, as it is said, heat becomes latent that is, the energy of the heat 

 which has become latent is changed into chemical energy, and this, after decom- 

 position of the compound chemical body, is again represented by the chemical 

 affinity between its isolated different atoms. 



LAW OF THE CONSERVATION OF ENERGY. Julius Robert Mayer and 

 Helmholtz have established the important law that, in a system which does not 

 receive any influence and impression from without, the sum of all the forces acting 

 within it is always the same. The various forms of energy can be transformed one 

 into the other, so that kinetic energy may be transformed into potential energy and vice 

 versa, but there is never any part of the energy lost. The transformation takes place 

 in such measure that, from a certain definite amount of one form of energy, a 

 definite amount of another can be obtained. 



The various forms of energy acting in organisms occur in the following modi- 

 fications : 



1. Molar motion (ordinary movements), as in the movements of the whole body 

 of the limbs, or of the intestines, and even those observable microscopically in 

 connection with cells. 



2. Movements of Atoms as Heat. We know, in connection with the vibration 

 of atoms, that the number of vibrations in the unit of time determines whether the 

 oscillations appear as heat, light, or chemically-active vibrations. Heat-vibrations 

 have the smallest number, while chemically-active vibrations have the largest 

 number, light- vibrations standing between the two. In the human body we only 

 observe heat-vibrations, but some of the lower animals are capable of exhibiting 

 the phenomena of light. 



In the human organism the molar movements in the individual organs are con- 

 stantly being transformed into heat, e.g., the kinetic energy in the organs of the 

 circulation is transformed by friction into heat. The measure of this is the " unit 

 of work " = 1 gramme-metre, and the " unit of heat " = 425*5 gramme-metres. 



3. Potential Energy. The organism contains many chemical compounds which 

 are characterised by the great complexity of their constitution, by the imperfect 

 saturation of their affinities, and hence by their great tendency to split up into 

 simpler bodies. 



The body can transform the potential energy into heat as well as into kinetic 

 energy, the latter always in conjunction with the former, but the former always by 

 itself alone. The simplest measure of the potential energy is the amount of heat 

 which can be obtained by complete combustion of the chemical compounds re- 

 presenting the potential energy. The number of work-units can then be calculated 

 from the amount of heat produced. 



4. The phenomena of electricity, magnetism, and diamagnetism may be 

 recognised in two directions, as movements of the smallest particles, which are 

 recognised in the glowing of a thin wire when it is traversed by strong electrical 



d 



