756 A TEXT-BOOK OF BOTANY. 



the object being to fix the contents in a condition approaching as 

 nearly as possible the normal living state. In order to carry out 

 this operation successfully, the living specimen must be placed in 

 the fixing or killing agent as soon as collected, and if the specimen 

 is large it should be cut into small pieces. The following are some 

 of the common fixing agents : Chromic acid in 0.5 to i per cent, 

 aqueous solution ; osmic acid in i to 2 per cent, aqueous solution ; 

 Flemming's mixture, which is an aqueous solution of chromic 

 acid (0.25 per cent.) containing o.i per cent, of osmic acid and 

 o.i per cent, of acetic acid; picric acid in concentrated aqueous 

 or alcoholic solution ; picric-sulphuric acid, a concentrated aqueous 

 solution of picric acid containing 2 per cent, by volume of sulphuric 

 acid; and mercuric chloride (corrosive sublimate) used in o.i to 

 i per cent, aqueous or alcoholic solution. 



HARDENING or DEHYDRATING AGENTS are those substances 

 which are employed for the purpose of hardening the specimen so 

 as to facilitate sectioning and for removing the water, which 

 would interfere with its examination. Alcohol is to be regarded 

 as the principal hardening or dehydrating agent, and considerable 

 care is necessary in its use; the specimen is treated successively 

 with alcoholic solutions of gradually increasing strength, begin- 

 ning with a 35 per cent, solution, in which the specimen is kept 

 for twenty-four hours ; then it is placed in 50 per cent, alcohol for 

 from six to twenty-four hours, and then in 70 per cent, alcohol, 

 in which it may be kept until ready for use. In order to avoid 

 shrinking of the material at this stage, it may be kept in a solu- 

 tion of alcohol and glycerin, or oil of bergamot, or a mixture of 

 xylol and paraffin. When the material is to be examined it should 

 be removed to 85 per cent, alcohol for from six to twenty- four 

 hours, then to 95 per cent, alcohol and absolute alcohol suc- 

 cessively for the same length of time. Of the other dehydrating 

 agents the most important are anhydrous glycerin, pure carbolic 

 acid, and anhydrous sulphuric acid, the latter being used in a 

 desiccator and not applied directly to the specimen. 



CLEARING AGENTS. Most dehydrating agents are also clear- 

 ing agents, because of the fact that the air and water in the speci- 

 men are replaced by a medium having greater refractive proper- 

 ties. Some clearing agents act chemically on the tissues and cell- 



