ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 603 



by two transverse pieces, and from the uprights proceed outwards in 

 opposite directions two branches of which there are several tiers. At 

 the end of each branch is a stop to prevent the slides from slipping oflf. 

 The distance between the branches is such that the ends of the slides 

 only just rest on them so that there is no fear of spoiling the preparations. 

 Rests of larger dimensions are made for sjiecial cultivations. For 

 this purpose the plates or slides are hollowed out on their upper surface 

 to jjrevent the nutritive medium from running off when liquefied by heat 

 or micro-organisms. These slides are 10 cm. long and 5 cm. broad, and 

 therefore can be examined with ordinary Microscopes without being 

 obliged to be turned round. Both rests are intended to be covered over 

 with a bell-jar. 



Nitric Acid in Gelatin.*— Dr. R. J. Petri, who recently showed the 

 presence of nitric acid and other adulterations in gelatin, now communi- 

 cates the cause of its presence. It appears that caustic lime is used in 

 the manufacture in order to get rid, by means of saponification, of any 

 fatty matters. The excess of lime is then removed by means of water. 

 But as a considerable quantity of lime still remains behind, this is 

 neutralized with nitric acid. Hence the jn-esence of nitrates and of lime. 



C6) Miscellaneous. 



Rosenbnsch's Petrographical Tables, an aid to the Microscopical 

 Determination of Rock-forming Minerals. — This is a translation,! by 

 Dr. F. H. Hatch, of Prof. H. Eosenbusch's ' Hiilfstabellen zur mikroskop- 

 ischen Mineralbestimmung in Gesteinen.' 



These useful tables contain, in handy form, convenient for reference, 

 the most important physical and chemical properties of the principal 

 rock-forming minerals. They constitute, in tact, a summary of the 

 contents of the first volume of Prof. Eosenbusch's ' Mikroskopische 

 Physiographic.' 



The tables are arranged under three main divisions, viz. Table I., 

 singly-refracting minerals ; Table II., containing two subdivisions a and 

 6; doubly-refracting uniaxial minerals; Table III., containing six 

 subdivisions a-f, doubly-refracting biaxial minerals. 



The several headings of the parallel columns forming each table are 

 as follows : — Name of mineral ; crystallographic system ; cleavage imder 

 the three divisions of quality, direction, and angle ; characteristic form ; 

 optical character (-}- or — ) ; principal zone or face ; forms and optical 

 character of principal zone ; colour ; pleochroism ; index of refraction (n) 



and double refraction under the four divisions n = ^ ^ , y _ ^^ 



o 



y8 — a, y — y8; optic orientation ; apparent optic axial angle ; dispersion ; 



specific gravity ; behaviour with reagents ; chemical composition ; 



remarks. 



New Method of Determining the Number of Micro-organisms in 

 Air. J — This new process, which was devised by Prof. T. Carnelly and 

 Mr. T. Wilson, is a modification of Hesse's method, in which a flask is 



* Centralbl. f. Bakteriol. u. Parasitenk., v. (1889) pp. 679-80. 

 t 4to, London (Swan Sonnenschein & Co.) n. d., 3 tables and preface. Cf 

 Nature, xl. (1889) pp. 313-4. 



t Proc. Roy. Soc. Lond., xliv. (1888) pp. 455-64 (1 %.). 



