102 
Fossil Wood and Lignites — Knowlton. 
exhibit superficially the slightest trace of organic structure, 
although careful microscopic examination of thin sections 
shows it to he generally present. It may, however, be split 
along certain lines, notable in a direction parallel to the medull¬ 
ary rays, where very plain structure shows superficially. 
Viewed as an opaque object, the outlines only of the wood 
cells and medullary rays are detected. 
Supposing a priori that all parts of this lignite must exhibit 
traces, at least, of organization, its intense blackness naturally 
becomes a serious obstacle in the way of a satisfactory examina¬ 
tion, since, in order to make a successful study with the higher 
powers of the microscope the specimen must be thin enough to 
be viewed by transmitted light. An attempt was made to grind 
down sections, after the usual manner of cutting rock sections; 
but, even when the sections were so thin as to begin to break 
in fragments and be torn from the slide, they still remained too 
opaque for even a ray of light to pass through. Other methods, 
as incineration, boiling in acids, etc., were equally unsuccessful. 
The method finally adopted, and which proved eminently suc¬ 
cessful, was that recommended by Griffith and Henfrey in their 
Micrographic Dictionary (2d Edition, p. 178) for the examina¬ 
tion of coal. The specimens are macerated for a week or more 
in a strong solution of carbonate of potash, u at the end of 
which time it is possible to cut tolerably thin slices with a 
razor. These slices are then placed in a watch glass with 
strong nitric acid, covered, and gently heated; they soon turn 
brownish, then yellow, when the process must be arrested by 
dropping the whole into cold water or else the specimens would 
be dissolved. The slices thus treated appear of a darkish amber 
color, very transparent, and exhibit the structure, here existing, 
most clearly.” The specimens are then best examined in 
glycerine, and may be mounted permanently in cells in this 
fluid. 
The translucency obtained by this process is brought about 
by the dissolving out of the hydrocarbons by the potash. This 
shows that there can be little or no free carbon present, else it 
CQuld not be dissolved by the liquids used. The intense yellow 
color produced is probably due to the presence of picric acid, of 
which, owing to its great coloring power, only a trace would 
be necessary. 
