REAGENTS. 799 



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 alco- 

 holic solution ; picric-sulphuric acid, a concentrated aqueous solu- 

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

 acid; and mercuric chloride (corrosive sublimate) used in 0.1 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. 



Making of Sections. — Sections of roots, stems, barks and 

 many fruits and seeds can be made directly without embedding 

 the material, and while sections can be made holding the material 

 in the hand, between the thumb and three fingers, the hand micro- 

 tome for holding the material may be used by those who are less 

 experienced. In the sectioning of leaves and other material that 

 is not firm, and fruits and seeds which are too small to hold in 

 the hand, the material should be embedded in some substance 

 which will hold it and give it stability. When the tissues are not 

 too hard the material may be placed between pieces of elder or 

 sunflower pith. In some cases the making of sections is facili- 



