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Fifthly. Veficular vapor confifts of a number of hollow 

 vifibie globules, highly eledrified *, and devoid of elafticity. It 

 forms a fort of middle ftate between water and invifible vapor, 

 and when in large quantity forms mijls and clouds j when in 

 fmall quantity it barely diminifhes the tranfparency of air. It 

 cannot fubfifl: for any time but in air faturated with invifible vapor^ 

 from whofe decompofition it arifes. 



Sixthly. The condenfation of vapor arifes not merely from 

 cold, but from cold and contiguity, otherwife vapor could not be 

 formed in a temperature below the freezing point. 



From this view of the nature of vapors, and the change they 

 produce in the weight and elafticity of the atmofphere, it is plain 

 that their prefence or abfence cannot fully account for the varia- 

 tions of the barometer. For if we fuppofe the atmofphere per- 

 fedly dry, the barometer at 30 inches, and the thermometer at 

 65°, and then a column thereof to be faturated with moifture, 

 its elaflicity being encreafed -s-V> it will contain -jV of its volume 

 lefs air than before faturation, fince the encreafe of its elafticity 

 arifes from the introduction of a new elaftic fluid amounting to 

 yV of its bulk : And fince the weight of the whole volume was at 

 firft equal to that of 30 inches of $, its weight will now be lef- 

 fcned by yV of 30 inches, that is nearly 0,59 of an inch. But 

 on the other hand it gained yV of its volume of vapor, therefore 

 its real lofs of weight will be the difference of the weight of yV 

 of air, and -jV of vapor j but the weight of air is to that of 



* II. Saufl". Voy. aux Alpes, p. 259. 



I 2 vapor 



