112 M. F. Mohr on the Nature of Heat. 



In the production of gas the same thing takes place. In 

 order to overcome the cohesion of water, some heat must be 

 destroyed and becomo imperceptible. In the coming back to 

 the liquid stato this heat must be again disposable and able to 

 put into vibration, i. e. to warm, the thermometer or the hand. 

 A gas vibrates so that its particles always tend to drive one 

 another further off. The mere presence of a gas therefore acts 

 like a material force. In a solid body the particles are by the 

 vibration not driven out of the sphere of attraction, in a liquid 

 body partly, and in a gas entirely. When one compresses a 

 gaseous body so as to make its particles move within this 

 sphere of attraction, they attract one another and become 

 liquid : — Liquefaction of gases by pressure. 



There is an essential difference between gases on the one 

 hand, and the two other states of matter on the other. 



In solids and liquids each temperature corresponds to a de- 

 terminate expansion, or each number of vibrations to a defi- 

 nite magnitude of their amplitude ; on the contrary, in gases 

 the endeavour is at all temperatures to indefinitely increase 

 these amplitudes. This endeavour shows itself as tension 

 (Spannung), and appears in gases only. The temperature is 

 the number of vibrations which a body makes in a given time ; 

 this number increases with elevation of temperature. All heat 

 which is not employed for this purpose disappears to our senses, 

 and thus we have the simple distinction between sensible and 

 latent heat : Sensible heat increases the number of vibrations ; 

 Latent heat increases their amplitude, or overcomes material 

 forces, i. e. destroys states of aggregation. 



When a mass of gas is suddenly compressed, the excursions 

 of the separate particles are diminished ; and thus some of that 

 force becomes disposable which before determined extent of 

 vibration, and was therefore not perceptible by our senses. 

 This force can now be employed only to increase the number 

 of vibrations, since their extent is limited ; that is, the gas 

 becomes warmer. Apparatus for inflaming tinder by compres- 

 sion of air. (Compressionsfeuerzeug.) 



When a gas is suddenly expanded, the separate particles are 

 compelled to make larger vibrations, for which greater force is 

 required. This can only be taken from the force which deter- 

 mines the number of vibrations (temperature), or from sur- 

 rounding bodies, i. e. the gas becomes cooled ; which experi- 

 ment verifies. Both phenomena are entirely unexplained by 

 the old view. 



Wide vibrations have greater force than narrow ; but at high 

 temperatures the vibrations always increase in extent. This 

 increase corresponds, for equal increments of heat, to a definite 



