the Plug- Cock Fog- Chamber. 363 



heat of vaporization, and pressure of" water (or other) vapour ; 

 let p, k, c, r denote density , specific heat at constant pressure, 

 specific heat at constant volume, and absolute temperature of 

 the air, the water-vapour contained being disregarded apart 

 from the occurrence of condensation. As above, let the 

 variables, if primed belong to the vacuum-chamber, otherwise 

 to the fog-chamber. Let the subscripts &c. also be similarly 

 interpreted, so that d is the known density of saturated 

 water- vapour at t degrees absolute. 



Assuming the law of adiabatic expansion to hold both for 

 gaseous water-vapour and for wet air in the absence of con- 

 densation, it is com-enient in a plug-cock apparatus of fog- 

 and vacuum-chamber (where p x is nearly given by p 3 ) to 

 reduce to adiabatic conditions, whence 



M*sr-e( ! iS')'v.-'.>. 



where m is the quantity of water precipitated per cubic 

 centimetre of the exhausted fog-chamber. Finally d, the 

 density of saturated water-vapour, must be known as far as t, 

 so that an equation d=f('r) is additionally given. Here tt u 

 the vapour-pressure at r l5 is usually negligible (about *5cm.) 

 as compared with p l} and p 1 may. in practice (where great 

 accuracy is not demanded) be replaced by p 3 , which, like p, 

 is read off, while ir holds at t, which is also read off. In 

 conclusion I give a numerical example taken from Table II. 

 for p' = 43*5 cm. 



If the original equation (isothermal) is taken, m = 5'36 x 10 -6 

 grams per cubic centimetre. If the above equation is taken, 

 ?>i = 5'35x 10~ 6 . If the above equation is taken and p t 

 replaced by p 5 , ??z = 5"30 x 10~ 6 , the error being 1 per cent, 

 of the true value, near enough in practice to admit of easy 

 correction. 



5. Computation of p 2 . — Since the plug stop-cock cannot be 

 closed before the water condenses in the fog-chamber after 

 sudden exhaustion, the pressure observed in the fog-chamber 

 when the room temperature reappears is smaller than j^. 

 An excess of air has passed to the vacuum-chamber, so that 

 the pressure within the fog-chamber is eventually p 2 , or less, 

 The equations for pi and p x ' remain as in § 3. 



