464 



SCIENCE 



[N. S. Vol. LIV. No. 1402 



cliroinate of itself is unable to diffuse in the 

 gelatin. The contrary is easily proved. More- 

 over, there are a great many precipitates that 

 give bands either in gelatia, agar, silicic acid 

 or even in filter paper and sand. It can not 

 be assumed, in every case, that one of the 

 reagents is fixed. Further, the facts quoted 

 by Dr. McGuigan in support of his hypothe- 

 sis are inaccurate. Bands of lead chromate 

 can be obtained in gelatin vpith the right 

 concentrations of lead acetate and potassium 

 dichromate, as also with silver nitrate in the 

 gel and the dichromate in aqueous solution. 



Examination of a great many different 

 kinds of precipitate in gels and other media 

 shows that band formation occurs only when 

 the precipitate is extremely finely divided, 

 or, practically, in the colloid state. If the 

 specific surface of the precipitate is insuf- 

 ficient there is no banding. The experiments 

 are made conveniently in test-tubes half 

 filled with gel on which the liquid reagent is 

 poured. As the specific surface increases, at 

 fijst, bands of denser precipitate are formed 

 in a diffuse column of precipitate extending 

 down the tube. With further increase of 

 specific surface, the bands become more 

 marked, until, eventually, there may be no 

 precipitate between. The formation of bands 

 in a diffuse precipitate absolutely disproves 

 the " supersaturation " theory. 



The attractive force, the effect of which is 

 well illustrated in Dr. McGuigan's photo- 

 graph, is that of adsorption. Wlien the pre- 

 cipitate is sufficiently finely divided it ad- 

 sorbs the solute from the adjacent zone of 

 gel. More solute diffuses into this zone from 

 the regions of gel more remote, where the 

 concentration of solute has not been dimin- 

 ished. But the solute is adsorbed as fast 

 as it arrives in the neighborhood of the pre- 

 cipitate and is removed from solution by the 

 excess of precipitating reagent. Thus a con- 

 centration gradient is set up towards the pre- 

 cipitate, and a considerable region of gel 

 adjacent to the precipitate becomes practically 

 devoid of solute. If the rate of diffusion 

 into the gel of the stronger reagent is suf- 

 ficient, this reagent will be able to travel 



through the exhausted zone until it reaches 

 a further region of gel where there is suf- 

 ficient solute to form another band of pre- 

 cipitate. The increasing distances apart of 

 the bands are due to the diminishing concen- 

 trations both of the solute in the gel and of 

 the reagent diffusing in. 



The specific surface of the precipitate is 

 influenced by the concentrations of the re- 

 action components, by the nature of the re- 

 action medium and by the presence of elec- 

 trolytes. Generally, it is determined by the 

 value of N in von Weimarn's somewhat in- 

 definite formula 



N = K.{P/L), 



where P is the excess concentration of the 

 substance to be precipitated, L its solubility 

 and K is a factor representing the viscosity 

 of the reaction medium and the physical 

 and chemical complexity of the reaction 

 components in solution. The formula is 

 being investigated further. But it has 

 been shown that the occurrence or non- 

 occurrence, of bands of a given substance in 

 different gels is due to the influence of the 

 reaction medium, and that, by varying its 

 specific surface, a substance can be obtained 

 in the banded form, or not, as desired. For 

 instance, silver chromate and dichromate 

 form bands in gelatin. In agar gel they oc- 

 cur as black ribbon-like crystals up to 

 several centimeters in length. By increas- 

 ing the specific surface of the precipitate in 

 agar, both salts have been obtained in a 

 banded form even more beautiful than in 

 gelatin. 



S. C. Bradford ■ 

 The Science Museum, 

 South Kensington, 

 London, S. "W., 



specialization in the teaching 

 of science 



To THE Editor op Science: It is somewhat 

 amusing to note Professor Gortner's reference 

 to the settee of science as if it were a thing 

 of the past, and then to find, on' an earlier page 

 of the same issue, an advertisement which calls 

 for a professor of zoology and geology. 



