On some Methods in Palceobotany . 113 
upon the object has the effect of differentiating its parts more clearly, 
of bringing out any slight differences of colour and of making minute 
veins or markings more easily visible. As is well known, water has 
a somewhat similar effect, but it quickly soaks in and often destroys 
the specimen, on the other hand the oil dries up slowly after use 
without damaging the specimen at all. This method was devised by 
Dr. T. G. Halle of Stockholm. 
The use of oil in the same way has proved very helpful in 
examining the polished faces of blocks of petrified material especially 
where the cell walls had lost their colour and were almost invisible 
without such treatment. 
The Collodion Method. 
In cases where the cast of the epidermal cells of a leaf have 
been retained in fine grained material such as tufa, the microscopical 
features of the original cells can often be made out by taking a 
collodion cast of the fossil. Accounts of this method have been 
published by Professor Nathorst 1 and Dr. Bather 2 . The process is 
very simple, a drop of collodion is placed on the surface of the fossil 
and allowed to dry; as it dries, it peels off the rock, and if sub¬ 
sequently mounted and examined under the microscope it may show 
clearly the outlines of the epidermal cells. I found great difficulty 
at first in employing this method successfully. The collodion dried 
slowly to a whitish semi-opaque film which often adhered firmly to 
the specimen and could only be removed with difficulty. This seems 
to be due to the composition of the collodion, and the ordinary 
British Pharmacopoeia solution is quite unsuitable. It apparently 
contains too much alcohol to allow the rapid drying of the film, but 
by allowing a quantity of this original solution to evaporate to a jelly 
and redissolving it in ether, a solution was obtained which dried 
rapidly, peeled off the rock readily and gave a transparent film. 
I have not found this method very useful when working with 
Palaeozoic and Mesozoic impressions because it is seldom that the 
matrix was suitable for retaining a cast of the epidermal cells. It 
is, however, of considerable value in dealing with petrified wood, 
etc. Films taken from freshly fractured surfaces, often show as 
much or more than a thin section cut in the same direction through 
the specimen. It is therefore of great use in a preliminary study of 
material and for comparison with sections. In order to get perfectly 
1 Ark. for Botanik VII, p. 1, 1908. 
2 Geol. Magazine, Decade V, Vol. V, p. 454, 1908. 
