PRIMARY fixatives: COAGULANTS 99 



DNA remains in solution (compare Levene -^^). Something similar 

 appears to occur in the fixation of tissues, for chromatin fixed by 

 picric acid has little affinity for basic dyes, while some forms of it 

 are rendered quite strongly acidophil. ^^ It must be supposed that 

 in these cases a basic protein is separated from DNA and coagu- 

 lated, while the DNA dissolves. 



Ejfects on the histological picture seen in parajfin sections. Even 

 with the help of an indifTerent salt, picric acid by itself is a grade 

 IV fixative. There is little tendency for cellular aggregates to 

 shrink away from one another, but cytoplasm generally assumes 

 a very coarsely reticular form, or contracts into a mass surrounding 

 the nucleus, or even disintegrates. Red blood corpuscles swell and 

 become spherical. The shape of nuclei is rather well preserved; 

 the nuclear membrane is clearly shown; a nuclear network is 

 produced. 



Compatibility with other fixatives. Picric acid is very tolerant of 

 mixture with other fixatives. It may be used with any of the others 

 described in this chapter and the next. 



MERCURIC CHLORIDE 



Standard concentration for fixation. Saturated aqueous solution. 



Formula and formida-weight. HgCU. 271-5. 



Description. The crystals are long, white needles. The molecule 

 has a linear shape, the chlorine atoms being at opposite poles of 

 the mercury atom. The melting-point is 275^ C, but the boiling- 

 point is not far above (301 ') and the substance sublimes easily. At 

 room-temperatures mercuric chloride dissolves in water at from 

 6-6 to 7*1% w/w. It is also readily soluble in ethanol and benzene. 



This substance is used as an antiseptic and disinfectant, in the 

 dressing of furs, in the intensification of photographic negatives, 

 and in the preparation of various mercuric compounds. 



Since mercuric chloride is usually prepared by sublimation 

 and in strong solution is corrosive to the tissues of the mouth, it 

 used to be called corrosive sublimate. When taken into the body 

 in small quantities it may cause acute nephritis, the tubules and 

 sometimes the glomeruli being affected; the formation of urine is 

 reduced and may cease, with consequent death. 



Ionization. The electrical conductivity of solutions of mercuric 

 chloride is low, because of limited ionization. Solutions of this 

 substance are extremely complex. Partial hydrolysis is said to 



