August 2, 1906J 



NATURE 



339 



designing a piece of apparatus which he calls the " optical 

 disc"; this, at a moment's notice, can be adjusted so as 

 to show the path of the rays in any one of the important 

 cases usually dealt with in elementary lectures on geo- 

 metrical optics. The reflection of a single ray, or a 

 number of rays (parallel or divergent) from a plane, con- 

 cave, or convex mirror ; the refraction of a ray at a 

 plane surface, including the case where total internal re- 

 flection occurs ; the path of a single ray or a number of 



"rays through a convergent or a divergent lens; the nature 

 of spherical and chromatic aberration ; the theory of the 

 rainbow, these are a few of the experiments which can 



■ be performed by its aid. The apparatus, which is sold by 

 Messrs. A. Gallenkamp and Co., Ltd., is very compact, 

 and its general arrangement is so good that one experi- 

 ment may be changed for another in about half a minute. 

 The same firm supplies an appliance comprising bent glass 

 rods, which show the total reflection phenomena generally 

 demonstrated by the aid of the illuminated fountain ; a 

 simple polarising apparatus, consisting of a pile of plates 

 and a black glass reflector, which may be attached to the 

 optical disc described above ; and a simple form of polari- 

 scope, together with specimens of strained glass showing 

 ihe characteristic coloured figures associated with double 

 refraction. 



.An interesting light is thrown on the difficult problem 

 of the behaviour of manures in soils by some recent 

 observations, made ujider the auspices of the Bureau of 

 Soils of the United States Department of Agriculture, by 

 Messrs. Oswald Schreiner and George H. Failycr, and 

 published in the form of two communications in the 

 Journal of Physical Chemistry (Nos. 4 and 5). One of 

 these deals with the absorption by different soils of the 

 phosphates of calcium and sodium from dilute solutions, 

 whilst in the other the removal by a soil of potassium 

 from an aqueous solution of potassium chloride is studied. 

 It is shown that the soils dealt with take up the phos- 

 phates and potassium from aqueous solution according to 

 the law of a monomolecular reaction, and that the action 

 is strictly reversible. Water washes out the absorbed 

 material according to a similar law. Each soil is charac- 

 terised by a definite limiting capacity of absorption, which 

 differs with different soils. It is remarkable that for 

 certain clay soils and clay loams this capacity is the same 

 for sodium phosphate as for calcium phosphate, pointing 

 to the occurrence of definite reversible chemical actions. 

 It is particularly noteworthy that when water percolates 

 through a soil the amounts of phosphate or of potassium 

 in the transmitted liquid give no clue to the quantities of 

 these materials present in the soil itself. The results are 

 determined, not by solubility alone, but by a special law 

 governing the removal of the absorbed substances. 



The Institute of Chemistry has published a " List of 

 Official Chemical Appointments held in Great Britain and 

 Ireland, in India and the Colonies." The list has been 

 compiled under the supervision of the proceedings com- 

 mittee of the institute by Mr. R. B. Pilcher, the secretary 

 of the institute, and its price is 2S. net. The list is 

 arranged in two main divisions ; the first contains appoint- 

 ments under the departments of State and professorial 

 appointments in the British Isles; the second section deals 

 similarly with India and the colonies. The information 

 provided indicates the steadily increasing demand for pro- 

 fessional chemical services in connection with State and 

 municipal administration, and it should prove of service 

 to chemists evervwhere. 



OUR ASTRONOMICAL COLUMN. 



Astronomical Occurrences in Aucust : — 

 Aug. 4. ih. Dm. Eclipse of Ihe Moon invisible at Greenwich. 

 ,, 9h. 3m. to loh. 13m. Moon occults 1 Capricoriii 

 (mag. 4-3). 



6. loh. Saturn in conjunction with Moon (Saturn 



o"49'N.). 

 ,, I2h. 21m. to I2h. 54m. Moon occults 1//' Aquarii 

 (niag. 4-5). 



7. Satuin. Major axis of rings =43" 82, Minor =3 "02. 

 10. I3h. iSm. to I4h. 13m. Moon occults |" Ceti 



(mag- 4'3)- 

 10-12. Epoch of the Perseid meteoiic shower. Radiant 



45° + 57°. 

 15. \enus. Illuminated portion of disc =0650; of 



Mars =0 997. 



18. I2h. 15m. Minimum of Algol fj8 Pcrsei). 



19. I3h. 13m. Eclipse of the Sun invisible at Green- 



wich. 

 21. gh. 4m. Minimum of Algol (B Persei). 

 24. Ilh. 47m. to I4h. 37m. Transit of Jupiter's Sat. 



III. (Ganynitde). 

 29. 6h. 29m. 10 7h. 43m. Moon occults C Sagittarii 



(m»g- 3'5)- 

 ,, Ilh. Mercury at greatest elongation (18° 11' W.). 

 31. i6h. 2m. to i8h. 54m. Transit of Jupiter's Sat. 



III. (Gan)mede). 



FiNLav's Co.MET (i9o6d). — The following approximate 

 eirments, corrected for planetary perturbations, are pub- 

 lished in No. 4106 of the Astronomischc Nachrichten by 

 M. L. Schulhof for Finlay's comet at the present epoch : — 



Epoch 1906 August 10 M.T. Paris. 



M = 354 22 457 



ir= 8 10 55'2l 



a = 52 22 377 V 19060 



'= 3 3 S'SJ 



<t>= 46 23 229 

 M = 542"-557 

 log (7 = 0-815560 



T = Sept. 73 (Paris) 



A daily ephemeris, from which the following has been 

 extracted, has been calculated by M. Fayet : — 



NO 191 8, VOL. 74] 



The brightness of the comet when discovered in 1886 

 is taken as i-o. 



The observation of this comet at Heidelberg on July 16 

 gave corrections of -12m. iis. and -1° 37' -5 to this 

 ephemeris, and thereby brought the calculated time of 

 perihelion 'to about September 8-35, igo6 (Paris M.T.). 

 .Applying these corrections to the above ephemeris, it will 

 be seen that the comet will be about 3° north of Mira 

 Ceti on August 4. 



An Unexplained Observation. — In No. 4106 of the 

 Astronomische Nachrichten Prof. Barnard places on record 

 the following observation, which he made so far back as 

 .August 13, 1892, and for which he has not vet been able 

 to find any explanation. While examining Venus with the 

 36-inch Lick refractor at oh. 50m. (G.M.T.) on .August 13, 

 1892, he saw a star of about the seventh magnitude in the 

 same field as the planet, and about i' south and 14s. + 

 preceding. The position of this object would be, therefore, 

 a = 6h. S2m. 30s., 5= + 17° u ' o ; this position reduced to 

 1855 becomes o = 6h. 50m. 21s., 5+17° i3'.6, and there 

 appears to be no such bright star in this place, neither does 

 it agree with the position of any B.D. star. The actual 

 elongation of Venus would exclude the possibility of the 

 unknown object being an intra-Mercurial planet, although 

 it does not preclude an improbable planetary body interior 

 to Venus. 



Although fourteen years have elapsed since the observ- 

 ation was made. Prof. Barnard has hitherto hesitated about 

 publishing the results, but now thinks that they should 



