1610 Journal of Applied Microscopy 



cover-glass with a pipette until the smeared surface is entirely covered. Allow 

 the blue to stain from one to five minutes, but the methyl or gentian violet will 

 ordinarily require but a few seconds. The time required for staining can be 

 quickly determined by a few trial preparations. Wash the cover-glass thoroughly 

 in water, absorbing all excess of water by blotting it between two pieces of filter 

 paper, dry it in the air and mount it on the slide, film side down, in a small drop 

 of Canada balsam. The preparation is now ready for examination under the 

 microscope. It should be first examined with the 2-3 and 1-6 inch objectives 

 and later with the 1-12 oil immersion. The above instructions apply to bacteria 

 in general. Some bacteria, however, are exceedingly difficult to stain, but when 

 once stained are exceedingly difficult to decolorize, and upon these peculiarities 

 recognition of certain forms is largely based. 

 Harvey Medical College. WiLLIAM H. Knap. 



MICRO-CHEMICAL ANALYSIS. 

 XIX. 



THE COMMON METALS.— SILVER GROUP. 



As a matter of convenience we can consider that this group comprises, as in 

 qualitative analysis in the wet way, the elements, silver, mercury in the form of 

 mercurous salts, lead, and the rare element thallium in the thallous condition. 



These elements, though widely separated in the Periodic System, are thus 

 grouped together because of the fact that their chlorides are of low solubility, 

 and are therefore precipitated more or less completely by dilute hydrochloric 

 acid or by solutions of chlorides. The other halogen acids and their salts also 

 yield precipitates with the members of this group. 



Because of their being precipitated by dilute hydrochloric acid, the micro- 

 chemical separation of the silver group from other elements would seem to be a 

 very simple process ; this, however, is not always the case, since there is not 

 infrequently a tendency toward the formation of double salts, etc., which tends 

 to complicate matters. 



Antimony and bismuth under certain conditions can be precipitated as 

 antimonyl chloride — SbOCl — and bismuthyl chloride — BiOCl ; more rarely 

 copper may separate as cuprous chloride — CuCl. In all three cases the 

 precipitate is soluble in a slight excess of the reagent. It must also be 

 remembered in this connection that concentrated hydrochloric acid will cause 

 the separation of not a few salts by forcing back their dissociation. Moreover, 

 the addition of hydrochloric acid to alkaline solutions may often produce a 

 precipitate of some compound held in solution by the alkali. 



SILVER. 



Of the many reagents which may be employed for the detection of silver, the 

 following will be found to answer all requirements : 

 I. Hydrochloric Acid. 

 II. Ammonium Bichromate. 

 III. Arsenic Acid. 



