PICRIC AND OSMIC ACIDS 485 



allowed to remain too long in it. The objects should lie well washed 

 out, after fixing, with alcohol, beginning with alcohol of 50 per cent, 

 or 70 per cent., and passing gradually to stronger alcohols. In order 

 to facilitate the removal of the sublimate from the tissues, the 

 alcohol should have added to it enough tincture of iodine to make it 

 of a good port-wine colour, and the objects should remain in it till 

 they themselves have acquired the same colour. They may then be 

 washed with pure alcohol, and further treated as desired. 



Solutions of sublimate, or the objects in them, must never be 

 touched with steel implements, as these produce at once precipitates 

 that may injure the preparations. To manipulate the objects, wood 

 or glass implements may be employed ; for dissecting them, hedge- 

 hog spines, or quill pens, or cactus needles. 



Tissues become of an opaque whiteness on fixation with sublimate, 

 which in the case of small transparent objects is a good guide for 

 controlling the duration of the fixing bath. The fixing action is 

 extremely rapid. 



Picric acid is a reagent that gives very fair results for general 

 work, and is especially to be recommended where great power of 

 penetration is required, as is the case in work with chitiiious 

 organisms. A saturated solution in water with the addition of 

 1 per cent, of acetic acid may be taken, or the picro-iiitric acid of 

 Mayer. This consists of water 100 parts, nitric acid of 25 per cent. 

 iS" 2 0.,, 5 parts, and picric acid to saturation. 



Objects should remain in these liquids much longer than in sub- 

 limate liquids ; for though the penetration is extremely rapid the 

 hardening power is slight. They may remain for twenty-four hours 

 without hurt, but in many cases three or four hours will suffice. After 

 fixation the objects should be brought into alcohol of 70 per cent, 

 (never water), in which they should remain for a few hours, and 

 then be transferred to alcohol of 90 per cent., in which they should 

 remain, the alcohol being frequently changed for fresh, until the 

 yellow tint of the picric acid has disappeared or at least become 

 greatly attenuated. Objects prepared in this way are best stained 

 in alcoholic staining solutions. 



Mixtures of picric acid solution with sublimate in various pro- 

 portions have lately been much vised, with good results. 



Osmic acid is a useful reagent for fixing small objects. It pre- 

 serves the forms of cells admirably, and at the same time imparts to 

 tissues a grey stain that is frequently of the greatest value in bring- 

 ing out delicate structures. This substance is sold in the solid state, 

 in sealed tubes containing from -j- 1 ^- grin, to 1 grin. It is extremely 

 volatile. Care should be taken to avoid exposure to the vapours given 

 off from it, as they are exceedingly irritating to mucous mem- 

 branes and may easily give rise to serious catarrh, conjunctivitis, &c. 

 Its solution in. pure water keeps very badly, as the slightest con- 

 tamination with any organic dust will cause it to reduce arid precipi- 

 tate. It is recommended, therefore, that only a small quantity be 

 kept in stock in the shape of aqueous solution, whilst another 

 quantity may be preserved in the shape of a 2 per cent, solution in 

 chromic acid of 1 per cent., or, better, in platinic chloride of the 



