442 



SCIENCE 



[N. 8. Vol. XLVI. No. 1192 



The cilia described in these Decapoda con- 

 form in every respect to all authentic descrip- 

 tions and pictures of cilia which have come 

 under the writer's observation. In many cases, 

 they are short and straight. In other in- 

 stances they are long and wavy. In still 

 other examples they cluster together to form 

 the so-called brushes. Furthermore, the cilia 

 in all the cases mentioned spring from a well- 

 defined border, and also contain the char- 

 acteristic basal granules. 



ISTathan Fasten 



Zoological Laboratory, 

 IjNiVEBSirT OP Washington, 

 Seattle, Wash. 



RHYTHMIC BANDINQi 



The formation of Liesegang's rings, known 

 sometimes as " rhythmic banding," is of in- 

 terest to the geologist and biologist as well as 

 to the chemist. The color arrangement of 

 agate is an excellent example of this phenome- 

 non. Liesegang's original experiments dealt 

 with the rhythmic precipitation of silver 

 dichromate in gelatine. A solution of silver 

 nitrate was poured on a solid gel containing 

 dilute potassium dichromate. The precipitate 

 of silver dichromate formed was not continu- 

 ous but marked by gaps or empty spaces at 

 regillar intervals. 



I found it possible to obtain distinct band- 

 ing of silver dichromate in loosely packed flow- 

 ers of sulphur. From this and other experi- 

 ments it is evident that a gel is not absolutely 

 necessary. In practise I found the best 

 medium for sharply marked bands to be silicic 

 acid gel. With this I secured remarkably 

 crystalline banding of mercuric iodide, as many 

 as forty bands in a test tube. Reduced gold in 

 red, blue and green colloidal particles recur- 

 ring in regular rainbow bands was obtained 

 with a special silicic acid gel. 



Basic gels made it possible to secure bands 

 of eupric hydroxide merging into red and yel- 

 low forms of cuprous oxide. In a silicic acid 

 gel of slightly basic reaction crystalline basic 

 mercuric chloride formed in very distinct 



1 Abstract of paper read at the Kansas City 

 meeting of the American Chemical Society, April 

 12, 1917. 



bands. The best banding in the absolute 

 clearness of the gaps was that of copper 

 chromate in a slightly basic gel. 



Upon these experiments a new theory may be 

 built. For illustration consider the copper 

 chromate banding. 



The gel contains a dilute solution of a 

 chromate and above it in the tube a solution of 

 a copper salt. The copper ions diffuse into the 

 gel, meet the chromate ion and form a layer 

 of insoluble copper chromate at the surface of 

 the gel. The chromate ions immediately be- 

 low this precipitation zone diffuse into this 

 region now depleted of chromate ions and 

 meet the advancing copper ions thus thicken- 

 ing the layer of copper chromate. According 

 to Fick's law of diffusion the rate of diffusion 

 is greatest where the difference in concentra- 

 tion of the chromate ions in two contiguous 

 layers is greatest, that is, just below the front 

 of this thickening band of copper chromate. 

 As a result the region near the band decreases 

 in concentration of the chromate ions faster 

 than the space below. Finally the copper ions 

 have to advance some distance beyond the 

 band to find such a concentration of chromate 

 ions that the solubility product of copper 

 chromate may be exceeded and a new band 

 formed. This repeats again and again. Of 

 course if the copper ions were retarded suffi- 

 ciently there would be time for the concentra- 

 tion of the chromate ions again to become 

 uniform throughout the remaining clear gel 

 and no gap would occur. Hence if the diffu- 

 sion of the copper ions is retarded by any 

 means the clear gaps decrease in depth — the 

 bands are closer together. If copper ferrocya- 

 nide bands are formed in similar manner they 

 almost merge after the first layer reaches a 

 thickness of a few cubic centimeters. Yet they 

 are distinct and agate-like. A precipitate of 

 copper ferrocyanide greatly retards the dif- 

 fusion of the ions that form it, hence we have 

 here the proper condition to reduce the clear 

 gaps to a minimum depth. 



The complete paper with working directions 

 and a full exposition of the theory will soon be 

 published elsewhere. Harry N". Holmes 



Obeelin College 



