698 



SCIENCE. 



[N. S. Vol. III. No. 71. 



doubtless the illumination and color of 

 water are frequently due to combined 

 causes. 



THE PLASTICITY OF ICE CRYSTALS. 



Dr. O. MiJGGE has recently published the 

 account* of a series of experiments on 

 he deformation of ice relative to its crystal- 

 line structure. McConnel's experiments 

 have shown that permanent deformation 

 by bending may be induced in an ice slab 

 only when the pressure acts in the di- 

 rection of the optic axis ; the optic axis re- 

 mains normal to the curved basal surface 

 after bending. 



Miigge shows that plastic translation 

 without bending is possible only in a plane 

 perpendicular to the optic axis. To the 

 middle of a small bar of ice placed across 

 two supports, the latter as near together as 

 possible, a heavy weight was attached by a 

 strap . The optic axis lay horizontal. A por- 

 tion of the ice, about as wide as the strap, 

 was gradually drawn down until completely 

 detached. (See figure.) The temperature 

 remained below freezing. The stretched 

 portions were optically oriented exactly 

 like the main bar, the axis lying every- 

 where horizontal as indicated by the arrows. 



The plane of the base was determined to 

 be the only plane in which such transla- 

 tion could be induced; pressure oblique to 

 a basal slab was found to produce torsion 



*Neues Jahrbuch fiir Min., Geol. und Pal., 1895, 

 Bd. n., Heft 3, p. 212. 



that tended to bring the optic axis into 

 coincidence with the direction of pressure. 



These experiments prove that plastic 

 deformation and flexibility are important 

 components of the movement of glaciers. 

 The parallel position of the optic axes of 

 associated ' Korner,' or glacial granules, 

 has been observed, at least locally; this is 

 undoubtedly due to the fact that by transla- 

 tion on planes parallel to the base and simul- 

 taneous bending, the optic axis is forced 

 into parallelism with the direction of pres- 

 sure. Observations on plates of ice cut 

 from the Aletsch Glacier show that where 

 its bed sharply slopes, the optic axis lies at 

 right angles to the lower surface of the ice. 

 It is probable also that the increased purity 

 of the ice at a glacier's lower extremity is 

 due to the gradual liberation of ' air bub- 

 bles ' in migration along definite planes. 

 T. A. Jaggar, Jr. 



Cahibeidge, Mass. 



NOTES UPON AGBICULTUBE AND EOBTI- 



CULTURE. 



THE POTATO SCAB. 



Several Experiment Stations are mak- 

 ing tests of various remedies for the po- 

 tato scab. This trouble of the potato is 

 due to a fungus closely related to the bac- 

 teria. 



Bulletin ISTo. 33 of the Rhode Island Sta- 

 tion gives a somewhat lengthy report of ex- 

 periments that cover three years with va- 

 rious chemicals. Dr. Wheeler and Mr. 

 Tucker, the authors, state that air slaked 

 lime, wood ashes and calcium carbonate, 

 calcium acetate and oxalate all increase the 

 scab ; while calcium chloride prevents it, 

 but likewise injures the potato plant. Cal- 

 cium sulphate (land plaster) is the only 

 form of lime not harmful to the potato which 

 fails to increase the scab. Common salt 

 reduces the amount of scab, and this ex- 

 plains why sea weed is healthful to potato 

 land when used for manure. Barnyard 



