70 



SCIENCE 



[N. S. Vol. XLVII. No. 1203 



the employ of tlie Sinclair Oil and Eefining 

 Corporation as geologist, has returned to the 

 university this year, but retains his connection 

 with the Sinclair companies. 



Mr. L. a. Eumsey, former instructor in or- 

 ganic chemistry at Iowa State College, has 

 been appointed head of the department of 

 chemistry at Denison University, Granville, 

 Ohio. 



Dr. E. K. Strong, of the University of Chi- 

 cago, has been appointed as professor of indus- 

 trial chemistry at the Oregon Agricultural 

 College. 



DISCUSSION AND CORRESPONDENCE 



RHYTHMIC PRECIPITATION 



The abstract of Dr. H. N. Holmes's paper, 

 read before the Kansas City meeting of the 

 American Chemical Society, April 12, 1917, 

 which appears in Science, November 2, 1917, 

 calls for some discussion. He proposes a 

 " new " theory to account for rhythmic pre- 

 cipitation bands. I have recently given a short 

 account of some of the earlier work in the 

 subject in a paper in the American Journal 

 of Science for January, 1917, from which it 

 is clear that the theory is comparatively old, 

 having been suggested twenty years ago by 

 Ostwald senior, and established six years later 

 by Morse and Pierce.^ Later workers have 

 agreed with these pioneers, and recently I have 

 shown that the rates of diffusion of the reag- 

 ents have to be taken into account in explain- 

 ing rhythmic precipitation, and that under 

 certain conditions bands which become suc- 

 cessively closer, or equally spaced bands, may 

 be produced. Morse and Pierce also showed, 

 fourteen years ago, that a gel is not essential 

 to the formation of precipitates in separated 

 bands, having obtained them in aqueous solu- 

 tions. It is of interest and importance that 

 Dr. Holmes has obtained them in loosely 

 packed flowers of sulphur. 



It might be asked what Dr. Holmes means 

 by " crystalline banding of mercuric iodide." 



1 Morse, H. W., and Pierce, G. "W., Zeitschr. 

 phys. chem., Vol. XLV., 1903, p. 589, or Physical 

 Beview, Vol. XVII., No. 3, September, 1903, p. 129. 



Is it possible that " banding of crystalline mer- 

 curic iodide" is meant? Again, it is difficult 

 to understand what is meant by "a thickness 

 of a few cubic centimeters," thickness usually 

 being measured in one dimension, not in 

 three dimensions. 



I would take exception to the statement: 

 " The color arrangement of agate is an excel- 

 lent example of the phenomenon." It may 

 possibly be an example of the phenomenon. I 

 have not studied agates in sufficient detail to 

 discuss the subject at this time, but such cur- 

 sory examinations of agates as I have made 

 have been sufficient to indicate that the off- 

 hand acceptation of agates as examples of 

 rhythmic banding by precipitation within a 

 medium of gelatinous silica is inadvisable. 

 There are very few agates which are not sus- 

 ceptible of other explanation. Liesegang, in 

 his " Geologische Diffusionen," after discuss- 

 ing agates as products of rhythmic precipita- 

 tion within gelatinous silica, is careful to 

 point out that he does not propose to apply 

 this explanation universally. 



It is unnecessary to state that the descrip- 

 tion of Dr. Holmes's experiments with silicic 

 acid gels will be awaited with interest. From 

 the partial account given in his abstract the 

 experiments would appear to be along similar 

 lines to those of Hatschek, and Hatschek and 

 Simon. J. Stansfield 



Geological Department, 

 McGiLL Univeesitt 



gravitational repulsion and the comet 



The results presented by the writer in a 

 paper recently published by the Academy of 

 Science of St. Louis^ may be of assistance in 

 explaining the behavior of the comsB and tails 

 of comets. Twenty years ago Newcomb gave 

 the following description in Johnson's Uni- 

 versal Cyclopaedia. 



When a bright comet is earefuUy examined with 

 a powerful telescope, a bow will sometimes be 

 seen, partially bent around the nucleus on the side 

 towards the sun. If watched from night to night, 

 this bow will be found to expand from the nu- 

 cleus, become diffused and finally lose itself in the 

 nebulosity of the coma. . . . These bows seem to be 



1 Trans., Vol. XXVIII., No. 5, November 8, 1917. 



