MICROSCOPICAL TECHNIC 423 



stance and nuclear structure; with sodium sulphate for the central 

 nervous system; with copper sulphate for bulky objects; with 

 sublimate, etc.). 



Nonelectrolytes. 



Alcohol. Although diluted alcohol causes shrinking, by em- 

 ploying absolute alcohol (not under 99.5 per cent) we obtain in the 

 case of compact structures (spleen, kidneys, digestive glands, etc.) a 

 fixation without shrinking. The explanation of this is found in the 

 double action of alcohol, both precipitating and chemical. The 

 latter which effects the transformation of sols and reversible gels 

 into irreversible gels requires a certain time and indeed more time 

 the more dilute the alcohol is, so that we must endeavor to hurry 

 the chemical action as much as possible by employing concentrated 

 alcohol. The double action of alcohol may be easily demonstrated: 

 If a solution of albumin is poured into alcohol a moderate amount of 

 precipitate is formed which dissolves again upon diluting with water; 

 the longer the time that elapses before diluting, the less is dissolved 

 and the further has the chemical coagulation process advanced. 

 Besides ethyl alcohol, methyl alcohol may be employed. 



Formaldehyd (Formol or Formalin). The 40 per cent formol solu- 

 tion in the shops is usually diluted 10 times with water; if we 

 speak of 10 per cent formol solution we mean that it contains 4 

 per cent of formaldehyd. Such a solution is preferable for uniform 

 fixation and preservation of compact organs (liver, brains); it is 

 less desirable for cell and nuclear structures. A 4 per cent formalde- 

 hyd solution is the best preservative for scientists on collecting ex- 

 peditions, even though it is not well adapted for fixation. After 

 formol fixation the staining is often not all that could be desired. 

 The preeminent properties of formol depend on the fact that it is 

 chemically very active, easily diffusible and hardly at all adsorbed 

 by organic substances. The chemical process of tanning which is 

 very similar to fixing, is much less complicated with formol than 

 with tannin and pyrogallic acid, with which an adsorption precedes 

 the chemical change. These two substances are hardly ever em- 

 ployed alone; at times they follow fixation with osmium. 



We have now reviewed the most important substances used as 

 fixatives. In practical histology almost all of them are used in 

 mixtures. We employ chromic acid + acetic acid, potassium bi- 

 chromate + sublimate + glacial acetic acid, nitric acid + potassium 

 bichromate, osmic acid + potassium bichromate, chromic acid + 

 picric acid + nitric acid, alcohol + glacial acetic acid, etc. Direc- 

 tions for fixing tissues are legion, but they are "cooking recipes" 



