Joly — On a Method of Determining Specific Gravity. 45 



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

 tracting (o 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 unaltered 

 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 Micrographique ", Fouque et Levy, p. 118). 



I have prepared no pellets approaching this density — 2- 7 7 — 

 but I prefer the use of this solution in all cases ; it seems to con- 

 centrate less rapidly by evaporation, and is more " creepy". It 

 should be preserved and reconcentrated by evaporation after use. 



In this operation of balancing it is advisable to use a camel's 

 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. If the mixed solutions containing 

 the pellet be left standing for some hours before finally adjusting, 

 it will be found on examination with a lens that bubbles will no 

 longer gather on the paraffin. Should it be desired to preserve the 

 adjusted solution for any little time, the final adjustment should 

 be effected in a stoppered bottle, otherwise concentration will occur 

 in a very short time on exposure to the air. 



The last operation is finding the specific gravity of this solution, 

 which gives us s in the formula. This is most accurately done in 

 a Sprengel tube, holding about 5 ccs. ; the bottle may also be used. 



The following Table records the results of ten experiments, 

 made in verification of the method. I have altogether made but 

 twelve experiments — one was spoiled by overheating and losing 

 some of the paraffin by overflow ; the other by inadvertently 

 touching with the heater, and thus drawing off a little paraffin. 



