Chemical Combination of Gases. 243 



rential equations (1) and (2), make observations before things 

 have got into a steady state. If the atoms could in any way 

 be withdrawn from the field of action as soon as they are 

 formed — if, for example, they combined with some solid body 

 which had no effect upon the molecules — then we could deter- 

 mine t, the paired time, by measuring the rate at which mole- 

 cules of the gas must flow into a vessel containing the solid 

 body and the gas undergoing dissociation, in order that the 

 pressure in the vessel should remain constant. For if m be 

 the number of molecules in the vessel, since the atoms are 

 absorbed as soon as produced, the rate at which the molecules 

 diminish is ra/t, so that if the pressure is to remain constant 

 molecules must flow into the vessel at the rate m/t; in this way 

 we could determine t. If the chemical action is slow, we 

 might find this quantity in another way. We have by equa- 

 tion (1) 



dn N-n _2n 2 . 



dt * t " t ; 



dn T -\r— 2<ft 



2l T 2t 



Integrating this equation Ave have, if K= {rN/2t + T 2 /lo't 2 J- 2 



(K+.+ J) 



or if t' be the time required to increase the number of free 

 atoms from ?i l to n 2 we have 



J_w( K+, " + ff)( K -(*' + f,)) =2 j; 

 2K ("-♦i)(*-(*+i))~ T ' 



Now K only involves the ratio of t to t, which we can find 

 by experiments on the steady state. Thus K is known, and this 

 equation will enable us to find r ; and since we know the ratio 

 of t to t, we shall be able to find t. 



§ 2. We can investigate in a similar way the case when 

 the molecules of two compound or elementary gases enter 

 into chemical combination without being previously decom- 

 posed, such, for example, as the case when PC1 3 and Cl 2 com- 



