Small Quantities of Dense or Porous Bodies. 31 



The disk removed is next weighed in a delicate balance. If 

 as small as described above, the balance should read definitely 

 to Q"2 mgr. Its weight is co in the equation. It is in all 

 cases manipulated by use of a clean ivory forceps. If very 

 minute it is weighed on a tared watch-glass, and so need not 

 be manipulated at all after preparation. Removed from the 

 balance, the small fragment (or fragments) of mineral is placed 

 upon the surface of the disk. The extremity of a slip of 

 copper, about 5 mm. wide, is now heated in a smokeless 

 flame — it is better to use a little copper ball, drilled and fitted 

 on to a fine steel knitting-needle — and held above the frag- 

 ment of mineral, care being taken not to approach it so closely 

 as to endanger the paraffin being volatilized or of its being 

 melted so far as to risk loss by running over. Preferably 

 the disk of paraffin should rest on a piece of clean copper; 

 this will keep the lower surface cool. In point of fact, the 

 mineral in general absorbing heat more freely than the paraffin, 

 melts the paraffin beneath it by conductivity, and there is little 

 risk of loss. The heating is continued till the mineral is 

 seen to be completely soaked with the paraffin — every crack 

 and cranny is then filled, the paraffin welling up and swallow- 

 ing the specimen and expelling all trace of air. 



When cold it is placed in the balance and weighed. By 

 subtracting co from the weight found, we have W, the weight 

 of the mineral. 



There is probably no loss of paraffin in this process. Thus 

 it will be found that if such a pellet be very carefully balanced 

 in a solution, removed, dried, and melted on the hitherto un- 

 altered face of the disk, and then replaced in the solution, 

 there is, if anything, a slight decrease of density ; on complete 

 cooling this decrease is inappreciable. 



The pellet is now dropped into a specific gravity solution. 

 A saturated solution of common salt and water (sp. gr. about 

 1*2) will in many cases be found sufficient to float it. If so, 

 we have merely to adjust by adding water. Otherwise we 

 resort to Thoulet's solution (Mineralogie Micrograiohique, 

 Fouque et Levy, p. 118). I have prepared no pellets ap- 

 proaching this density (2*77), but I prefer the use of this 

 solution in all cases ; it seems to concentrate less rapidly by 

 evaporation, and is more " creepy. " 



In this operation of balancing it is advisable to use a 

 cameFs-hair brush for stirring, and also for conveying small 

 quantities of liquid when finally adjusting — a process of 

 much delicacy. The brush is also used for removing bubbles 

 from the pellet, which, however, will be found to give little 

 trouble if the solutions be previously boiled to expel air. 



