Chemical Combination of Gases. 247 



might see some reason for a remarkable result obtained by 

 M. Friedel, viz. that the effects when the methyl oxide was 

 in excess were slightly different from those when the hydro- 

 chloric acid was in excess. For it is probable that the effects 

 of a collision, when a molecule of a compound is struck by 

 a molecule of methyl oxide, will not be the same as if it 

 be struck by a molecule of hydrochloric acid ; so that the 

 effect of the Collisions produced when the methyl oxide is in 

 excess will be different from those produced when the hydro- 

 chloric acid is in excess. 



If there are any solid or liquid substances which unite 

 directly with oxide of methyl or hydrochloric acid, we 

 can determine the values of t and i by the first method in 

 § 1. We can also determine these quantities if we can find 

 any solid or liquid that will combine with the compound 

 formed by the combination of these gases, and yet not with 

 the gases themselves. For suppose we have a quantity of 

 this solid in a vessel filled with a mixture of equal parts of 

 oxide of methyl and hydrochloric acid, and that the pressure 

 in this chamber is kept constant by allowing oxide of methyl 

 and hydrochloric acid to stream in at a measured rate. Then, 

 since N = M, the rate of formation of the compound is W/r; 

 and as it is absorbed by the solid as soon as made, the rate 

 at which either the methyl oxide or the hydrochloric acid 

 must stream in, in order to keep the pressure constant, is 

 N 2 /t; if we measure this quantity, since we know N we can 

 find t. If we can use neither of these ways we must try 

 the second method of § 1. 



The dissociation of the pentachloride of phosphorus PC1 5 

 into PC1 3 and Cl 2 is another example of the case we are con- 

 sidering; and in the following table the vapour-densities at 

 different pressures calculated by equation (5) are compared 

 with the vapour-densities at those pressures observed by 

 M. Wurtz (Comptes Rendus, lxxvi. p. 601). The vapour- 

 density of the normal pentachloride is taken as 7*2, and the 

 observed and calculated results are made to agree at the 

 pressure of 243 millimetres of mercury. 



Dissociation of Phosphorus Pentachloride at 137° C. 



Pressure in 



Vapour 



-density. 



■milliTtipfvA^ nf 





r v 



Mercury. 



Calculated. 



Observed. 



281 . . 



. 6-51 



6-48 



269 . . 



. 6-49 



6-54 



243 . . 



. 6-46 



6-46 



234 . . 



. 6-455 



6-42 



148 . . 



. 6-28 



6-47 



