Mar., 1889. micro-chemical examination oe minerals. 
59 
MICRO-CHEMICAL METHODS FOR THE 
EXAMINATION OF MINERALS.* 
BY MR. T. H. WALLER, B.A., B.SC. 
The difficulties of the geologist who undertakes the 
examination of rocks in thin slices by means of the microscope 
are so great, and the determinations at which it is possible for 
him to arrive are so frequently based on evidence which it is 
a matter of extreme difficulty to present to others whom he 
may wish to convince, that he is sure to hail with delight any 
fairly simple and accurate means of settling by actual analysis, 
even though it may be on a very minute scale, the composition, 
and hence the nature, of the mineral components of any rock 
he may have under his observation. Optical methods of 
research have of late years made extraordinary advances. The 
apparatus for observing the effects of a minute crystal section 
on converging polarised light, and hence of coming to a 
conclusion as to the position of the optic axes of the crystal, 
whether the crystal is uniaxial or biaxial, and whether the 
double refraction is positive or negative, enables us in many 
cases to discriminate between minerals which present a very 
similar aspect when reduced to thin slices— e.g., quartz and clear 
felspar. That purely optical methods do not, however, always 
lead different observers to the same results, may be seen from 
the correspondence as to the nature of a certain constituent of 
some of the serpentines of the Lizard district which appeared in 
the Geological Magazine ” last year. And yet the difficulties 
in the way of an analysis are considerable. Of course where 
the crystals are of such a size as to be separable by careful 
breaking out, and are in sufficient quantity, a quantitative 
analysis by the ordinary methods of the laboratory is possible, 
and is the best possible procedure. Where the “ grain ” of 
the rock is moderate, it is still possible, when the mass has 
been crushed to a tolerably even fineness, to separate the 
constituents by means of some of the heavy salt solutions 
which are prepared for the purpose, such as the double iodide 
of mercury and barium, or of mercury and potassium, and 
make a chemical examination of the different parts. By this 
means most valuable results are obtained, and it is not 
necessary to do more than refer to almost any thorough 
examination of a rock published in the geological journals 
to show the importance of the procedure. 
* Read before the Birmingham Natural History and Microscopical 
Society, October 18, 1887. 
