REAGENTS. 19 



potash, the suberin escapes in the form of yellow viscid 

 drops ; when the sections are only slightly warmed in 

 potash solution the cuticularised cell-walls assume a 

 yellow colour. 



Potash is also used, together with copper sulphate 

 as a test for proteids, and for various kinds of sugar. 



Ammonia.. The solution in water is often used 

 instead of potash for clearing preparations, as its action 

 is less intense. It is used, together with nitric acid, as 

 a test for proteids, and with copper sulphate as a solvent 

 for some forms of cellulose. 



III. Non-Metallic Elements. 



Iodine. This is .one of the most useful micro- 

 chemical reagents. It is used in solution, in water, or 

 alcohol, and in the chloride of zinc mixture. 



1. Solution in water. Dissolve a small quantity of potassium 

 iodide in the requisite quantity of water ; then dissolve iodine in 

 it until the liquid has a dark sherry colour. This may also 

 be prepared by diluting the liquor iodi of the pharma- 

 copoeia. 



2. Alcoholic solution. Dissolve iodine in alcohol until it has a 

 dark sherry colour. This may also be prepared by diluting with 

 alcohol the tinctura iodi of the pharmacopoeia. 



Iodine stains proteid substances brown, cellulose 

 faintly yellow (as a rule, see next chapter), cuticularised 

 and lignified cell-walls yellow, gum purple, starch blue 

 (only in the presence of water). 



Iodine is used as a micro-chemical test for starch and 

 for cellulose. The blue colour which it gives with 

 starch, and the conversion of the faint yellow colour of 

 a cellulose cell- wall stained with iodine into blue when 



c 2 



