INTRODUCTION TO FIXATION 17 



of them, when mixed with a solution of albumin, produce a 

 coagulum: others do not. The ones that produce a coagulum in 

 a test-tube transform homogeneous protoplasm into a micro- 

 scopical spongework. 



Against each fixative in the list below is marked its 'standard 

 concentration'. This is the concentration at or near which it is 

 commonly used in fixation. Throughout the book, except where 

 the contrary is distinctly stated, it is to be understood that when 

 reference is made to one of the seven selected primary fixatives, it 

 was used at the concentration shown in the list (or at a concentra- 

 tion so close to this that no diff"erence in result could be 

 anticipated). 



COAGULANTS 



Ethanol (ethyl alcohol). C0H5OH. A light, colourless fluid, 

 miscible with water in all proportions. Standard concentration, 

 undiluted (absolute). 



Mercuric chloride. HgCL. Colourless crystals, soluble in water 

 at about 7%. Standard concentration, saturated aqueous 

 solution. 



Chromium trioxide. CrOg. Brownish-red crystals, giving a 

 strongly acid solution in water, in which it is extremely soluble. 

 Standard concentration, 0-5% aqueous. 



NON-COAGULANTS 



Formaldehyde. H2CO. Colourless gas, very soluble in water. 

 Standard concentration, 4% aqueous. 



Osmium tetroxide. OSO4. Pale yellow crystals, soluble in water 

 at about 7%. Standard concentration, 1 % aqueous. 



Potassium dichromate. KoCraOy. Orange-red crystals, giving a 

 weakly acid solution in water, in which it is soluble at about 

 10%. Standard concentration, 1-5% aqueous. 



Acetic acid. H3C.COOH. Colourless liquid, miscible with 

 water in all proportions. Standard concentration, 5%. 



Since each of the primary fixatives has its virtues and defects, 

 c 



