NOTES AND MEMORANDA. 468 



pellicle composerl of a more or less compact mass of Bacteria, en- 

 veloped in a hyaline, transparent substance of slight consistence. This 

 membrane is so fragile that the slightest movement or breath which 

 ripples the surface of the water tears it. A tolerably large piece of 

 this membrane can be obtained by carefully introducing into the liquid 

 beneath it a glass slide, and raising it with caution. 



If we then add, wdth a pipette, one or two drops of a concentrated 

 solution of osinic acid (or even a solution of 1 in 100) to the mem- 

 brane on the slide, it immediately acquires a much greater consistency 

 and can be covered without fear of tearing it. A drop of a solution 

 of violet of methylaniline should be placed at the side of the cover- 

 glass, drawing away the osmic acid by a cigarette paper on the 

 opposite side. In about half an hour the Bacteria assume a fine violet 

 tint, the fundamental siibstance remaining colourless ; if the impreg- 

 nation lasts longer the Bacteria assume a deeper hue, and the funda- 

 mental substance becomes tinted. We can then replace the violet of 

 methylaniline by glycerine, which does not render the preparation 

 colourless, as Koch says, if we add a small quantity of the violet. A 

 concentrated solution of sulphate of calcium can also be used with 

 advantage to preserve the preparations. M. Blanchard's collection 

 contains preparations made thus in 1876, which are as bright in colour 

 as at first. 



The violet is not the only aniline colour which can be used, but 

 it seems to be more durable than others. 



A solution of hfematoxylin can also be used with advantage. 

 When a "proliferous membrane' (F. A. Pouchet) has been treated 

 with osmic acid, it is left for twenty-four hours under a damp bell- 

 glass, in a watch-glass containing a few drops of hjematoxylin. There 

 is then formed an iridescence which spoils the clearness of the prepa- 

 ration, but which can be easily removed by repeated washings. The 

 membrane is then mounted in glycerine (with or without the addition 

 of hfematoxylin), or in a solution of chloride of calcium, and preserves 

 indefinitely a fine violet tint. 



If the Bacteria are free in the liquid, the process of mounting 

 them would be exactly the same. 



To prepare Infusoria, or any of the lower organisms, osmic acid 

 should be used, but in a strong or even concentrated solution which 

 instantly kills the animalculfe. A group of Vorticella thus fixed will 

 retain their natural form, some of them being completely extended and 

 others more or less retracted. Amoebfe, Ehizojioda, &c., have no time 

 to retract their protoplasmic filaments, and die spread out on the glass 

 in their living aspect. 



Ciliated Infusoria do not lose their cilia, and except a slight 

 blackish hue they are in no way modified by the reagent. Some 

 Opalinfe found more than a year ago in the intestine of a Triton have 

 preserved to this day the delicate cilia with which their body is 

 covered. 



The contact of the osmic acid must not be prolonged, or the objects 

 will blacken with age. After the animalculfe are covered with the thin 

 glass, a few drops of picro-carmine or htematoxylin can be added. 



