364 Efficiency of the Plug-Cock Fog- Chamber. 



The new quantities are 



Pi i \(A— e) 7./ \c & 



Pi 



^ -7T= —{p -IT) { Pl -TTi) , 



Pi 



where J3 T is the density of air at t^ The ratio pi/pi may be 

 found when t 1 is known as 



Pi = (^/-^--(^/-^^(^--TrOCl + p/Y.TxVT ,) 

 Pi Ol— w-J (^/Tj + r/V . Tj'/tJ 



where T\ and t/, 7r/ and 77-/, p 1 and j^' are nearly the same. 

 The last equation may usually be written 



and the small quantity involving the vapour-pressures it 

 treated as a correction. It amounts to about 1 per cent of 

 the large quantity. The values of p 2 are also given in the 

 table and chart. This shows that jp 2 observed is always 

 smaller than p 2 computed, even when allowance is made for 

 the condensation of water ; ?', e. the fog-chamber begins to 

 appreciably heat itself above the temperature ti before the 

 cock can be closed again, so that when isolated the fog- 

 chamber contains less than its proper quantity of air. Only 

 the initial and the final (both chambers communicating) 

 pressures may therefore be taken at the fog-chamber. 



6. Conclusion. — If the fog-chamber is combined with a 

 large vacuum-chamber, through a sufficiently wide passage- 

 way containing an ordinary plug gas-cock to be opened and 

 closed rapidly by the hand, all the measureable coronas of 

 cloudy condensation, due to the presence of colloidal nuclei 

 in wet strictly dust-free air, may be evoked. While such an 

 apparatus admits of capacious fog-chambers and extremely 

 simple manipulation, it has not been shown to be inferior in 

 efficiency to any other apparatus whatever. 



The conditions of exhaustion must, however, be computed 

 from the initial pressures of the fog- and vacuum-chambers 

 when separated and their final pressure (after exhaustion) 

 when in communication, all at the same temperature, and 

 the volume ratio of the chambers. The chief pressures and 

 temperatures are shown in the figure for different initial 

 pressures of the vacuum -chamber, the fog-chamber being at 

 atmospheric pressure. 



Brown University, 

 Providence, R. I. 



