1821.] Causes of Calorific Capacity, Latent Heat, #c. 267 



Theory of the Nature and Laics of Vapours.* 



The intimate physical cause of the conversion of fluids into 

 vapours being closely connected with the cause of the reflection 

 of lio-ht at the anterior surfaces of bodies, the limits of the pre- 

 sent paper would not allow me to develop it in a manner suitable 

 to its extent and importance ; for which reason I have thought 

 it better to omit touching on the subject in this paper altogether. 

 And as it might have seemed strange to treat of the cause of conden- 

 sation without explaining that of vaporization, and I perceived 

 it would be necessary to say something of the nature and laws of 

 vapours, I preferred deferring my observations of condensation 

 to this place when they might serve to throw some light on the 

 nature and laws of vapours. 



Were tire particles of an air sufficiently small, it would, per- 

 haps, be impossible by any means, short of total abstraction of 

 motion or temperature, to produce a union or condensation when 

 they meet ; for the force of condensation diminishes as the 

 magnitude of the particles diminishes, while the force to resist 

 which is as the temperature may remain the same. But one of the 

 chief causes of condensation is the irregularity and adaptation of 

 figure. Spheres of all figures are the least adaptable, and the 

 most efficient to resist a union. When bodies with irregular 

 figures meet, the chance is many to one against their striking 

 each other in a line passing through their centres of gravity ; 

 consequently, the collisions will generate a whirling kind of 

 motion about these centres of gravity, which will occasion the 

 bodies sometimes to strike with a greater, and sometimes with a 

 less, than their mean force. By this means it must happen, not- 

 withstanding the mean motion of the particles be the same, that 

 they will recede with a less force after some collisions than after 

 others, and hence will be less able to resist a union in those 

 cases than in these. This inability to oppose a union will like- 

 wise be increased or diminished according as the most adaptable 

 sides are turned towards or from one another at the times of the 

 collision, and according as the particles are then moving towards 

 the same or opposite parts. Let us, therefore, conceive that at 

 a certain temperature the force of collision is sufficient to over- 

 come the most favourable circumstances of union, but at a less 

 temperature it is not. Then at all higher temperatures the air 

 would preserve its gaseous property, but at inferior temperatures, 

 when the concomitant circumstances were favourable, the parti- 

 cles would not be able to recede with a force equal to that 

 resulting from the adaptation of their striking parts, and they 

 would, consequently, unite; and this union successively running 

 through the parts of the air will compose an air with less in num- 



Thii theory will receive a further elucidation when I come to speak of the phat- 

 Aofncna and laws of evaporation. 



