THE FELSPARS. 
221 
necessarily tend to increase the percentage of that alkali, and 
correspondingly diminish the lime. It is much to be regretted that 
Professor Heddle has not made (or published, if he has made) 
observations on the microscopical and optical properties of the grand 
series of felspars analysed by him. They would almost certainly have 
afforded an immensely improved point of departure for argument as 
to the actual chemistry of the group. 
To determine what felspar we have in a rock is, unfortunately, a 
very difficult task, at any rate where it occurs in but small-sized 
crystals. Chemical analysis is very difficult in the case of such 
small quantities of material as are usually available, and is 
frequently but uncertain on account of the almost unavoidable 
admixture of other minerals. But two or three methods have been 
proposed, and of them I propose to speak, though only very briefly, 
seeing that to properly elucidate the subject I should have to make 
more experiments before you than there is time and opportunity for 
to-night. I the less regret this, as some time ago I read a paper, and 
showed experiments, on one of the methods mentioned. 
This, which is the one elaborated by Dr. Szabo, professor in the 
Univerisity of Buda-Pesth, is, in essence, a carefully arranged deter¬ 
mination of the fusibility in different parts of a Bunsen burner flame 
of particular (or at least invariable) dimensions,‘the same operations 
also serving for the estimation of the percentage of the alkalis by the 
intensity of the flame-colouration. This method, which can be com¬ 
pletely carried out in about a quarter of an hour, if all goes fortunately, 
enables us to decide at once and easily between Orthoclase, Albite, 
Oligoclase, Labradorite, and Anorthite, but requires more practice 
and more careful observation to determine accurately the varying 
proportions of soda in Orthoclase (in eighteen analyses of Scotch 
felspar, by Dr. Heddle, this varies between O’o3 and 5*5%), the division 
of Andesine between Oligoclase and Labradorite, and the occurrences 
between this last and Anorthite, which have been called Bytownite. 
The best way is to compare the specimen with fragments of known 
composition, one on one side of the flame, the other on the other. 
Another plan is that of Dr. E. Boricky, depending on the facts that 
a dilute solution of fluosilicic acid decomposes silicates, and that the 
fluosilicates of several of the bases which occur most frequently in 
minerals crystallise in characteristic forms, and so can be detected, 
after the drying up of the drop of reagent, by means of the microscope. 
Thus Orthoclase leaves beautiful cubic and octohedral crystals of the 
Potash salt, and a few hexagonal prisms of the soda one. In the case 
of Albite the proportion of the two sorts of crystals is reversed. 
For the purpose of separating portions of felspar for trial by either 
of these methods it is very convenient to use the heavy solution of 
iodide of Hg. in iodide of K. which Sonstedt proposed, and which is 
now being much used, especially in Germany, to get out the various 
constituents of a rock for purposes of analysis. The solution can be 
got of a sp. gr. of just over three, so that felspars and quartz float 
