Plant Impressions or Incrustations. 383 
put into an etching bath of hydrofluoric acid (HF) and the rock etched 
away. If there is any effervescence on immersion in the HF the preparation 
should be removed at once, as effervescence indicates the presence of 
a carbonate, probably of calcium, in the rock. If the reaction were allowed 
to proceed insoluble calcium fluoride (CaF 2 ) would be produced. If 
calcium carbonate (CaC 0 3 ) is present dilute hydrochloric acid (HC 1 ) must 
be used as a mordant until all the CaC 0 3 is removed. The preparation 
must then be washed thoroughly to remove all traces of soluble calcium 
salts, and if any matrix insoluble in HC1 is left it will probably be silica or 
silicates, which may then be removed by treatment in the HF bath. 
The etching is continued until the matrix is-dissolved or is loosened 
sufficiently to be washed away by a gentle stream of water. The prepara¬ 
tion should not be left too long in the HF as the balsam is acted on very 
slightly. 
It is often instructive to watch the process of etching by taking the 
slide out of the bath from time to time, washing, and then examining under 
the microscope, as portions of other plants embedded in the rock may be 
seen which are removed in the course of the etching. 
8. The preparation is washed in water. 
9. The wax is cut away and any excess of balsam on the slide trimmed 
off if necessary, 
10. It is sometimes advisable to warm up the preparation on the top 
of a steam oven until the balsam is soft; this ensures that the plant is 
firmly fixed. Before it is warmed every trace of wax must be scraped 
away or it may flow over the surface of the balsam and spoil the preparation. 
11. The resulting preparation may be covered with hot fused balsam 
transferred to it on a cover-slip, but there is danger of the plant moving 
and cracking up if this is attempted. It is generally sufficient to keep the 
preparation free from dust, and for examination under the microscope 
glycerine in water can be put on the surface, or some other liquid with 
a refractive index as near as possible to that of balsam, but which will 
not dissolve or react chemically with it. 
Examples. 
In order to demonstrate the possibilities of this method of preparation 
in the elucidation of fossil plant structure I give a series of examples of 
different types of preservation of carbonized impressions from horizons 
ranging from the Devonian to the Cretaceous. In conjunction with 
Schulze’s method of isolating cuticles the investigation of Coal Measure 
plants is rendered easier; for after the plant has been transferred the 
balsam may be dissolved away subsequently and large coherent portions 
obtained for cuticle preparations. The more uncertain procedure of chipping 
off small pieces is thus avoided. 
C c 2 
