sequent peels may be made with a 25% solution acting 
for two minutes. The first etching frequently makes the 
specimen more susceptible to subsequent attacks. 
Partially silicified specimens may be etched the first 
time with 25% hydrofluoric acid and thereafter by 10% 
hydrochloric acid for two minutes. 
Hydrofluoric acid must be used with great caution 
because of its dangerous corrosive action, especially on 
the mucous membranes and on the fingernails. 
Coals require special softening treatment before they 
can be etched. This will be discussed later. 
Etching is atrial and error process. All kinds of pet- 
rifaction occur and frequent trials may be necessary before 
a suitable concentration is secured. Under ordinary cir- 
cumstances the etched surface is carefully washed with 
running water in order to remove excess acid. 
Drying. The etched specimen must be air-dried in a 
dust-free place. If one is making a number of peels, it is 
frequently helpful on a humid day, to set in motion an 
air current by means of a small electric fan. After the 
specimen has thoroughly dried, it is placed in a level po- 
sition on plastilene bases or in a sand-box. 
Smearing. The etched surface must be carefully cov- 
ered with a nitrocellulose solution. The solution can be 
applied by two means: it may be poured gradually over 
the surface by a continuous slow flow from a bottle, or 
it may be smeared by drawing a glass stirring rod over 
the specimen. We pour the solution, because fewer air 
bubbles form on the surface. Bubbles may be punctured 
with a dissecting needle or removed with a scalpel. If, 
upon drying, the film is too thin to allow removal, ad- 
ditional solution may be poured on the surface and then 
permitted to dry. 
Various kinds of solutions have been used by individ- 
ual investigators. The following formula will give good 
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