Mar., 1889. micro-chemical examination of minerals. 
63 
and especially at the edges of the very characteristic needles 
or blades of gypsum, showing the proper angles of the 
clinopinacoid of the crystals, and occasionally the arrow-head 
twin which is so frequent among the larger natural crystals. 
Behrens speaks of this test as of extreme delicacy, and says 
that it is capable of showing ^cho m 8 T - lime. To another 
drop add at one edge a fragment of bisulphate of potash, 
unless you feel inclined to pay for the corresponding salt of 
caesium, which, however, is decidedly more delicate as a test. 
In presence of alumina the octohedra of potash (or caesia) 
alum will begin to appear almost at once, and are quite 
umnistakeable. As the potash alum only contains about 10 
per cent, of alumina, and the caesia salt only about 9 (the 
latter, moreover, is very much less soluble in water), the 
delicacy of the reaction is considerable. In cases where 
potash and alumina are both present in the mineral under 
examination, as e.g ., in the case of an orthoclase, or a rock 
containing orthoclase, the presence of both bases may be 
shown by the crystallising out of the alum along with the 
needles of gypsum. 
At a distance of about J of an inch from another of the 
drops place a drop of a solution of chloride of platinum, and 
allow the drops to run together and gradually mix. The 
presence of Potash will be shown by the almost immediate 
appearance of the double chloride of platinum and potassium 
in orange vellow octohedra. variously modified about the 
angles. Where only minute proportions of potash are 
present the distinguishing crystals may only appear on the 
partial evaporation of the drop. If the bases have been 
dissolved as chlorides of the various metals, the reaction for 
potash is more rapid, but the crystals are less perfectly 
shaped. 
To test for Soda we may use either the acetate of uranium, 
as suggested by Streng, or sulphate of cerium, as preferred by 
Behrens. The former reagent must be added to the dried 
residue of a drop of the solution of the bases slightly 
evaporated, and observed on cooling. In the presence of soda 
distinct yellow tetraliedra of the double acetate of sodium and 
uranium separate. They contain less than 6 per cent, of 
soda, and therefore show a very minute quantity of this base, 
but the test cannot be applied in the presence of free 
sulphuric acid, even of a mere trace, and is also liable to be 
obscured by the formation of crystals of the acetate of 
uranium, or of a basic acetate. A little practice, however, 
will enable very good results to be obtained. 
In solutions of the sulphates the addition of a concentrated 
solution of cerous sulphate (the yellow ceric sulphate is of no 
