the Plug- Cock Fog -Chamber. 351> 



At the beginning (case 1), the fog-chamber is at atmo- 

 spheric pressure, p (nearly), the vacuum-chamber at the low- 

 pressure p', and both at the absolute temperature t. On 

 suddenly opening the stop-cock the adiabatic pressures, &c, 

 given under no. 2 appear, supposing that no condensation 

 has yet taken place in the fog-chamber. I£ the stop-cock 

 could now be suddenly closed and the whole apparatus 

 allowed to regain the original temperature t, the conditions 

 under no. 4 would obtain. This is virtually the case in 

 Wilson's* piston apparatus, and consequently these variables 

 are comparable with his results, cf. § 3. In the present 

 apparatus, however, condensation takes place within the fog- 

 chamber before the stop-cock can be closed, and thus an 

 additional quantity of air is discharged from the fog- into the 

 vacuum-chamber. After condensation and before the stop- 

 cock is closed, the conditions under no. 3 apply ; when the 

 stop-cock has been closed and the apparatus thereafter 

 allowed to regain the room temperature t, the conditions are 

 shown in no. 5, and may be observed (with crude approxi- 

 mation) in the isolated chamber. Finally when the chambers 

 are put in communication, the variables, no. 6, are the same 

 in both. 



This account of the phenomena may seem prolix ; but 

 it is essential to a just appreciation of the efficiency of the 

 plug-cock fog-chamber. Quantities in the above table 

 referring to a given chamber may be connected at a given 

 time by Boyle's law, as for instance, p—n=RpT. This gives 

 eleven equations, some of which may be simplified. Corre- 

 sponding quantities in groups 1 and 2, as for instance 

 t/tj, may be connected by the law for adiabatic expansion 

 giving two equations. In addition to this, an equation 

 stating that a given mass of air is distributed in fog- and 

 vacuum-chamber (volumes v and Y, respectively) is avail- 

 able ; or 



vp + Yp' ■= vpi + Ypi = &c., = (r + Y)/3 3 . 



All the quantities ir are supposed to be given by the 

 corresponding t, though at high exhaustions the lower limit 

 of known data, ir =/(t), is often exceeded. 



3. Computation of p 1 and p^ — It will first be necessary to 

 compute p 2 , the pressure which w r ould be found in the fog- 

 chamber when it has again reached room-temperature r, if 

 there were no further transfer of air from fog-chamber to 

 vacuum-chamber, due to the condensation of water-vapour in 

 the former after adiabatic cooling. The pressure p 2 is given 



* C. T. R. Wilson, Phil. Trans. London, vol. 192, pp. 405 et seq. 

 (1889). 



