VIII. THE IONS. IIS
great as the gas pressure at the some temper&tnre, the osmotic pressure being due to the impacts of the dissolved molecules against the semi-permeable membrane. For dissolved oxygen in aqueons solution at 20° the velocity of the molecnle is therefore also 425 met/sec.
The dissolved molecules collide with the molecules of the solvent, and ultimately the mean value of the kinetic energy per gram-molecule will be the same for each, l^e velocity of the liqnid molecules of the solvent must be the same as that in the gaseous state, therefore the velocity of water molecules in both conditions will be 566 met./sec., and the molecules of the solid shotild have the same velocity. The mean velocity increases proportionally to the square root of the absolute temperature.
Now, all molecules of one species do not possess the same velocity ; thus, all water molecules at 20° do not have the velocity 566 met./s9C., but this number represents the average value (1 in Fig. 24), and most of the velocities lie near this (Maxwell). Velocities ranging from to very high values occnr. However, the further any particular velocity is from the mean value the smaller is the number of molecules which possess this velocity, as is shown in the accompanying diagram. Amongst the water molecules there are therefore to be found some with a
velocity, e.<f. 3 times as great ' ' '
as the mean velocity, and this corresponds with the mean velocity at a tempera- ture 9 times higher than the 4- 20°), i.e. at the tempe- rature 2364°, At this I^h temperaturewaterispartially Fia. 24.
dissociated into hydrogen
and oxygen molecules, which indicates that the water mole- cules cannot withstand the rapid motion at this temperature without partly decomposing. A small part of the water at
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