78 Domestic Science 



and their movements are not confined to any particular 

 neighbourhood. The shape of a given mass of liquid 

 is therefore easily altered by external agencies. 



The molecules of gaseous substances have almost 

 entirely lost the property of cohesion. They are 

 supposed to move in straight lines with great rapidity, 

 colliding frequently with one another and the walls 

 of the containing vessel. The motion of any given 

 molecule is practically unaffected by the presence of 

 adjacent molecules. Their great freedom of motion in 

 all directions carries them into all parts of any con- 

 taining vessel, however large, the lack of cohesion 

 between them permitting of their ready separation from 

 one another. 



Let us now apply this hypothesis to the explanation 

 of what has been observed in studying gaseous diffusion. 

 The molecules of bromine and those of the various 

 constituents of the air being in a state of constant 

 movement in all directions as described above, some 

 molecules of bromine, by virtue of this motion, will 

 leave the topmost layer of vapour and mingle with the 

 molecules in the layer of air immediately above. On 

 the other hand, some of the molecules in the lowest 

 layer of air will leave that region and pass into that 

 part of the jar formerly occupied by bromine. This 

 process of intermingling will be comparatively slow, in 

 spite of the rapidity with which the molecules move, 

 for the collisions of the latter with one another and 

 with the inside of the jar will cause them to be deflected 

 into new paths with great frequency, and thus only 

 part of the time will be occupied in moving in directions 

 which will carry them into new regions. If a sufficient 

 interval of time be allowed to elapse, however, the 

 continuance of the intermingling process will at length 



