1821.] Causes of Calorific Capacity, Latent Heal, Sfc. 365 



£) e f 4. — Ponderatom, from ponderis, of weight, and atomi, 

 atoms, implies the number of atoms in a unity of weight, or the 

 number of atoms whose aggregate mass of matter is unity. 



Def. 5. — Megethmerin, from pzyeQoe, of volume or magnitude, 

 and fjupi:, a particle, denotes the number of particles in a unity of 

 volume. 



Def 6. — Baromerin, from /3«&o;, of weight, and p;pi;, a particle, 

 signifies the number of particles in a unity of weight, or the 

 number of particles whose collective mass of matter is unity. 



Def 7. — When in the following part of this paper I speak of 

 the megethmerin of a vapour, I mean the number of vaporous 

 particles in a unity of volume, not taking into account how many 

 or how few, nor whether there be, or be not, any gaseous parti- 

 cles mixed with them. 



Prop. VII. Theoe. V. 



If two portions of the same fluid, having equal depths, be 

 equally and similarly exposed at the same temperature, the 

 quantities evaporated will be directly proportional to the areas of 

 the exposed surfaces. 



Fcr since the depths and temperatures are equal in both, no 

 irregularity can arise from any inequality on either of these 

 accounts ; and since the fluids are the same, and equally and 

 similarly exposed, there can be no inequality in the momentary 

 evaporating action on like parts of the superficies ; therefore, 

 the evaporating influence, to whatever cause it may be owing, 

 being the same on equal parts, the ratio of the whole evaporating 

 actions, and, consequently, the ratio of the whole quantities 

 evaporated in any small particle of time will be equal to the 

 ratio of the evaporating superficies. But if throughout any 

 small portion of time the evaporating causes on equal superficies 

 be equal, they will be equal throughout the next portion, and so 

 on ad infinitum ; and, therefore, the whole quantities evaporated 

 for any length of time indefinitely, will have the same ratio ; that 

 us, the ratio of the superficial arcs. 



Cor. — This proposition is mathematically true only when other 

 things being alike, the depths are equal. But if the depths are 

 unequal, but so great that the evaporations for any given time 

 will not sensibly affect the general temperatures of the fluids, 

 the ratio of the quantities evaporated will still have very nearly 

 the same ratio as the evaporating superficies. 



Prop. VIII. Tiieor. VI. 



If any portions of the same fluid be cooled by evaporation 

 alone, from any common temperature to any other common 

 temperature, the quantities of fluid lost by evaporation will be 

 directly proportional to the quantities of the liquid ; and con- 

 versely, if any two portions of the same fluid lose quantities pro- 

 portional to their weights by evaporation, these losses, if their 



