IN GASES: HYDROGEN, CARBONIC OXIDE, AND OXYGEN. 
665 
60° to 120°. The cause of the change must be sought in some condition present 
up to 60° and absent at higher temperatures. There is such a condition—the 
condensation of the steam by the sides of the vessel during the progress of the 
chemical change. By this condensation one of the reacting bodies is removed from 
the sphere of action. If we conceive the gases to be composed of a vast number of 
molecules moving in straight lines and coming into collision one with another and with 
the sides of the vessel, and that an appreciable time elapses during which the gaseous 
molecules are in sufficiently rapid motion to change atoms when they come into 
collision, then it must happen that while the change is proceeding a number of steam 
molecules strike the sides of the vessel. Now a vapour diffused through a gaseous 
mixture is condensed to a liquid by a cool surface when the pressure of the vapour in 
the mixture is greater than the tension exerted by the liquid at the temperature of 
the cool surface. On the contrary, when the surface is at such a temperature that a 
liquid film upon it exerts a tension greater than the pressure of the vapour of that 
liquid present in the atmosphere in contact with it, the liquid volatilizes from the 
surface. Under the latter conditions no vapour would be condensed from a gaseous 
mixture. For a surface at a given temperature, condensation depends upon the 
tension of the vapour in the mixture, and for a given tension of the vapour, condensa¬ 
tion depends on the temperature of the surface. 
These facts permit the question to be readily tested by experiment. In the first 
series of temperature experiments, if 37 be taken as the constant coefficient at high 
temperatures, then in the last experiment the tension of the steam formed by the 
unimpeded reaction would be 162 millims. at 60°. At this temperature the tension 
exerted by a film of water is about 150 millims. During the explosion, therefore, the 
sides of the vessel might condense a small quantity of steam, and the coefficient might 
in consequence be slightly raised. In the previous experiment at 40°, the tension of 
steam formed by the unimpeded reaction would still be 162 millims. at 40°; but at 
this temperature the tension exerted by a film of water is only 55 millims. So that 
a considerable condensation might take place, and the coefficient be considerably 
raised. At lower temperatures the effect would be greater still. In the same way, if 
37 be taken as the constant coefficient in the second series of temperature experi¬ 
ments, then in the second, third, fourth, and fifth experiments the tension of steam 
formed by the unimpeded reaction would be 152 millims. at 60°, 80°, 100°, and 120° 
respectively. At 80° the maximum tension of aqueous vapour is 355 millims., so that 
at 80° and at higher temperatures no condensation could take place; but at 60° a slight 
condensation might occur. The two series of experiments show therefore that the 
coefficient remains constant when no condensation is possible during the reaction : 
with decrease of initial temperature the coefficient begins to increase as soon as con¬ 
densation becomes possible. Similarly in the third series of experiments, if we take 
4 as the constant coefficient, no condensation could take place at 80° or higher tem¬ 
peratures ; at 70° some condensation might occur. The coefficient begins to increase 
MDC'CCLXXXIV. 4 Q 
