738 SUMMARY OF CURRENT RESEARCHES RELATING TO 



difficulties connected with it. Examples of these difficulties are : — 

 the time required for the reaction, the formation of gelatinous pulver- 

 ulent white crusts of aluminium silicofluorides which hide the minute 

 crystals of the other silicofluorides, especially the very transparent 

 sodium salt, and then the calcium, iron, magnesium, and other 

 fluosilicates are very soluble, and crystallize only when the solution 

 dries up completely. Owing to these difficulties, and the want of 

 methods for detecting silica and alumina, the author was led to look 

 for other methods more convenient, quicker, and having a wider 

 application. 



Preparation of the Test Sample. — If the individuals composing the 

 rock are larger than 1 • 5 mm., then small fragments may be broken 

 from splinters of the rock by means of a pair of pliers ; their homo- 

 geneous nature is tested by a lens, or a low power under the Micro- 

 scope. If the rock is of a finer grain it must be crushed and the 

 dust removed; using a low power small fragments of any of the 

 constituents may now be picked out for examination. If the con- 

 stituents are such as not to be readily distinguished from each other, 

 when coarsely powdered, a section must be made of the rock, but no 

 thinner than is necessary to give sufficient transparency for examina- 

 tion under a magnifying power of a hundred diameters ; the top surface 

 of the section may be either slightly polished or smeared with 

 glycerine or oil. The balsam is softened by a gentle heat, and the 

 required fragments picked out under the Microscope with a needle or 

 knife, using a low power and a high eye-piece, and freed by ignition 

 from balsam, &c. The selected fragments are ground in an agate 

 mortar. They are brought into solution by means of a very little 

 fuming hydrofluoric acid, or of ammonium fluoride and strong hydro- 

 chloric acid (this is done in a small platinum spoon) and then gently 

 evaporated to dryness ; the residue is moistened with a small quantity 

 of dilute sulphuric acid and heated till most of the free sulphuric 

 acid is removed. Water is then added, and the whole gently boiled 

 until but little more than one drop remains. This solution of the 

 sulphates is taken up by a capillary glass tube of • 2 mm. diam., 

 and in this manner divided and placed on slides for examination by 

 the tests for the various substances. The solutions are examined 

 without cover-glass, since it allows of better and quicker working ; 

 the objective is protected by a small plate of mica fastened on with a 

 drop of glycerine, a power of 150-250 diams. is most convenient. 

 The weight of substance required is from 0*2 to 0*5 milligram. 



Calcium. — If any considerable quantity is present gypsum begins 

 to crystallize out at once in short prisms, or if in smaller quantity 

 then after a few minutes as crystals of the usual form of gypsum, 

 oo P. P. oo P. oo P. oo oo . Mean size 0*060 mm. If but a trace of 

 calcium is present it may be detected by allowing the drop to absorb 

 a little alcohol vapour, the gypsum then separates in needles. 



Potassium. — To the preceding test-drop is added a drop of platinum 

 chloride solution, by means of a loop of platinum wire. The double 

 salt soon separates ; if not, it may be accelerated by the action of 

 alcohol vapour. It forms very sharp light-yellow octahedrons, with 



