C. Barns — Drop of Pressure in Fog Chamber. 339 



Art. XXXII, — Note on the Actual Drop of Pressure in the 

 Fog Chamber • by C. Barus. 



1. The apparatus for condensation, as I have endeavored to 

 use it, consists of a fog chamber in communication with a 

 vacuum chamber through a wide stop-cock. The former may 

 be put in connection with the filter ; the latter with the air 

 pump. It is necessary to wait between operations, all observ- 

 ing being done at the same temperature. In this case the 

 isothermal value of the drop of pressure cannot be read off at 

 the fog chamber (as I supposed it could, nearly), however 

 rapidly the cock is closed after exhaustion ; but it may be 

 computed from the initial pressures of the isolated fog and 

 vacuum chambers before exhaustion, and the final pressure 

 when the vessels are in communication after exhaustion, if the 

 ratio of volumes of the vessels is known. 



2. Let v be the volume of the fog chamber, V the volume 

 of the vacuum chamber, k/c the ratio of specific heats of the 

 gas (moist or dry as required) ; let p, v, t, p, denote its pres- 

 sure, volume, absolute temperature and density under condi- 

 tions given by the subscripts. It will be convenient to refer 

 to the vacuum chamber by the same symbols with accents. 

 Hence the thermal states will be for dry air. 



For the For the 



Fog chamber. Vacuum chamber. 



Initially p r p p' t' = t p 



Adiabatically (alone) _ p 1 r 1 p l p^~p^ T / p/ 



Isothermally (alone) _. p 2 t 2 = t p. 2 — p l p 2 ' T 2 '— T P 2 —P\ 



Isothermally (together) p\ t=t p 3 P?=P, t 3 '^t 3 pl'=p. 



The equations describing the transformations are (for dry 

 air) 



r Ti) = <p/p*Y k - 



-c)/k ( T / T /) = (2J l2\Y k - c)/k 



(1) 



p — Rpr 



Pi z=Jip i T i 



p,= Xps 



p 3 = izpj 



p' = Ep'r 

 P'i= R P\ T V 



P-2 = UPS 0r 

 PJP\ = P-2 1 P-2 



(8) 



p v-tp' V= Pi v+p\ V= p 2 v+ P ' 2 V=p 3 {v+ V) (3) 

 From these one may deduce relative to the value of p x 



e/k _ P*(l+v/V) 



yj '•//■' — £_n — : — I : (4) 



2l p'(k-c)/k) + ( v I y} p(k-c)/k \> 



p\+P,.v/Y=p 3 (l+v/V) (5) 



