July 22, 1921] 



SCIENCE 



79 



of precipitation are excluded, this supersatur- 

 ated condition may persist for a considerable 

 time without the spontaneous development of 

 a solid phase. Such solutions he calls meta- 

 stahle. By the diffusion of the silver salt into 

 the chromate gelatine, a solution is formed 

 which in relation to the silver salt is super- 

 saturated. A precipitate is formed only after 

 the metastable limit has been exceeded. This 

 jDreeipitate occurs naturally in zones con- 

 centric with the drop. On the precipitate 

 that is formed the silver supersaturated 



The siimllcr lijfure {A} sliowa the sphere of 

 influence of the silver nitrate. The larger figure 

 (B) shows the efEeot of the smaller when included 

 in the larger. Note that the smaller sphere has 

 removed the chromate so that the rings in the 

 larger are interrupted. 



chromate in the region lasts until all the sol- 

 uble silver is precipitated. Then the silver 

 salt wanders out over the ring into the 

 chromate gelation until a new supersaturated 

 region is formed and the precipitation process 

 is repeated. 



The main objection to Ostwald's explana- 

 tion is that a supersaturated condition has 

 been shown unnecessary for ring formation; 

 also that there are other factors involved in 

 the ring formation.* Since no explanation 

 has been accepted, I wish to present one which 

 seems adequate. 



The chromate in the gelatine is relatively 

 fixed and diffuses very slowly; when AgNOj 

 is added, there is an immediate formation of 

 silver chromate not only under the silver so- 



lution but there is a sphere of influence which 

 can be seen with the aid of a hand lens (see A 

 figure). The silver attracts the chromate from 

 this area and leaves it sharply demarcated. 

 This demarcation could be due to the with- 

 drawal of the chromate or it might be due to 

 the influence of the potassium nitrate formed 

 in the reaction. However, the amount of po- 

 tassium nitrate that could be formed has no 

 such influence on the gelatine chromate; and 

 an experiment can be devised to show that 

 there is no chromate, or very little of it, in 

 this zone. The experiment is as follows : Place 

 a minute droplet of silver nitrate on a gelatin 

 plate until the zone of influence is distinct. 

 Then at a short distance from it place a large 

 drop of silver nitrate (.B) sufficiently large so 

 that when the Liesegang rings are formed 

 they will include the smaller drop. After a 

 time a condition develops as shown in figure 

 where the larger circles are interrupted by 

 the zone of influence of the smaller particle. 

 This shows that there is not enough chromate 

 to precipitate; it has all been attracted by 

 the first silver nitrate. The explanation of 

 Liesegang ring formation then is as follows: 



Silver chromate is formed and a clear zone 

 results in the gelatine by the attraction 

 of the chromate to the silver. Beyond this 

 zone of influence, the chromate is fixed and 

 remains so unless an attraction force is ex- 

 erted. The silver nitrate now wanders out 

 through the ring into the clear zone until it 

 approximates the chromate gelatine sufficiently 

 close to exert an attraction force which again 

 draws the chromate and forms another ring 

 and clear zone. At the same time the chro- 

 mate exerts a pull on the silver and the ring 

 is formed where the forces are balanced. 

 Again it may be presumed that to start the 

 chromate moving, will require a greater force 

 than to keep it moving after the start is naade, 

 consequently the second ring is separated from 

 the first. 



With each succeeding ring the concentration 

 of the silver is less and this will also operate 

 to remove the succeeding rings farther and 

 farther apart. Ring formation by these or 

 by other reagents depends on or is modified by 



