( 413 ) 



atniosphei'C of llie suiTOiin(lin<>- inodiiini. And f'urllioriiiorc, wliethcr 

 carbon nutrition takes place exclusively in the floating dry funis, 

 — hence, in the earth, only on the relatively dry snrfjxce of the 

 earth ])ariicles, — ■ or that also in the de|)th of fluids growth and 

 carhon assimilation be possible. The hitherto gathered experience 

 about the self-purification of I'ivcrs and the biological purilication 

 of water in general, seems to exclude the latter iiy|)othesis, and 

 our own experiments too, render it not probable. The result of 

 these experiments consists, in our opinion, in the very discovery of 

 a microbe, which, in consequence of the film-formation, has thes|»e- 

 cific faculty, to absorb for its initrition and nudtiplicatiou. from ;i 

 gas, namely the air, traces of volatile carbon compoimds, by Nvhich 

 the struggle for existence with the rest of the microbic world can be 

 successfully sustained. The biological purification of water would, 

 according to this view, find a counterpart i]i the biological pin-ilic- 

 ation of the air by JJacil/if.s' oliyocarhonhilu,'^ 



Physics. — "The calculation of — fjvm the inai/iH'ff'c rotation of 



til ' 



the phiite of polarisation , for suhMances icitlioiit an absorption 

 band in the visible spectrum." By Dr. L. H. Siertsema. (Com- 

 munication No. 82 from the Physical Tjal)oratory at Tjciden 

 by Prof. H. Kamerlingh Onnes). 



Startmg from Fitzgerald's^) simple explanation of the magnetic 

 rotation of the plane of polarisation derived from the Zeeman effect, 

 and also from the supposition that the result of the magnetic force 

 is only shown l)y the displacement of the dispersion curve of the 

 medium (;n=f{?.)) over a distance d, Hallo') finds for the magnet ic 

 rotation w 



2 jr dn 



where z represents the thickness of the medium. Hallo's investigations 

 are concerned with the parts of the spectrum in the neighl>ourhood 

 of an absorption band and for these we are justified in making the 

 above supposition, as appears from a formula derived by Yokjt 



^) Fitzgerald. Proc. Roy. Soc. 63 p. 31. 

 2) Hallo. Diss. Amsterdam 1902, p. 7. 



