OXIDES AND OXYGEN ACIDS OF SULPHUR 263 



evolved. This tendency, if unchecked, would lead to a decreased 

 yield of sulphur trioxide; the cold entering gases are therefore 

 first led over the outside of the pipes which contain the catalyst, 

 in order to keep the temperature constant inside. 



The issuing gases, consisting mainly of sulphur trioxide vapor 

 mixed with excess oxygen, are condensed by being led into 97-99 

 per cent sulphuric acid, the concentration of the liquid being 

 maintained at this point by a regulated influx of water. If 

 oleum, or fuming sulphuric acid, is required, the addition of water 

 is omitted. 



It would seem to be simpler to dissolve the gaseous sulphur tri- 

 oxide in water, to give sulphuric acid H^O + SOs > H 2 SO4, rather 

 than in 98 per cent sulphuric acid, but this cannot be done. The 

 mixture O 2 + 2S0 3 is very incompletely absorbed by water. 

 When a bubble of this mixture enters water, the latter evaporates 

 into the bubble in the attempt to saturate the space occupied by 

 the bubble with water vapor (p. 62). The water which so evap- 

 M> orates, however, combines immediately with the sulphur trioxide 

 6f to form a fog, consisting of droplets of liquid sulphuric acid, and 

 a so more and more water evaporates into the bubbles. Now the 

 cules of SO 3 , so long as they remain gaseous, move with great 

 velocity, namely 292 meters per second at room temperature, 

 and still faster in this hot gaseous mixture (see p. 90). Hence, 

 all the molecules that escape combination with the water vapor, 

 strike the wall of the bubble, and combine with the water in a few 

 seconds. The droplets of sulphuric acid, forming the fog, how- 

 ever, are not molecules but large aggregates of molecules. They 

 do not therefore move like the molecules of a gas, but are rela- 

 y stationary. The chance of their striking the wall of the 

 bubble is therefore reduced enormously. Hence, after the sulphur 

 trioxide that escapes combination has dissolved, the droplets of fog, 

 carried by the excess of oxygen, can be bubbled through a whole 

 series of vessels of water in succession without any appreciable num- 

 ber of the droplets being dissolved. The same fog can be shaken in 



