^ot Elutriation of Emiry. 



(6.) In order to obtain very fine emery, and of .different degrees of finenefs, tlie 

 fieve is not fufBcient, but water niuft be ufe4. After having weH ground the coarfe 

 emery on an iron plate with a muller of the fame metal, it is thrown into a veffel which 

 muft be rather wider at bottom than at top, by a gradual increafe. The vefTel muft 

 then be filled with clear water fo that it may ftand eight or ten inches above the emery. 

 The whole is then to be ftrongly agitated with an iron fpatula; after which it 

 rouft be left to fettle for an hour. The emery falls to the bottom, but the water remains 

 turbid, being charged with emery or fome other extremely fine and light matter. Into 

 this water muft be plunged the fhorteft branch of a fyphon filled with clear water to the 

 depth of four in€hes below the. furfac'e of the water, holding the other extremity of the 

 fyphon clofed with the finger, which is afterwards withdrawn that the water may flow 

 through the fyphon without ftirring the vefTel or agitating the emery which is at the bot- 

 tom. The water thu» drawn off, is to be received in another large vefTel, and the firft 

 veffel is to be again filled, and its contents agitated as before. The fame operation is to 

 be repeated till the water paffes clear through the fyphon. The powder which has 

 paffed over with the water and was received in the fecond veffel is too fine to be ufefully 

 employed in grinding glafs. The veffel being emptied and cleared, the fame operations 

 are to be repeated, with this difference only, that inf^ead of leaving the water to fettle for 

 an hour, no longer time is employed than half an hour, and when the water paffes clear 

 through the fyphon, the operation is difcontinued, and the emery obtained by fubfidence 

 from the water, and carefully defended from all impurities, is referved under the deno- 

 mination of emery of half an hour. 



The fame operations being repeated, allowing only a quarter of an hour for the fubr 

 fidence, affords an, emery, which is indeed fine, but lefs fo than the preceding, and is to 

 be referved under the denomination of emery of a quarter of an hour. 



By fimilar proceffes emery may be obtained of half a quarter of an hour, of four, of 

 two, of one, of one half, or one quarter of a minute. To meafure the half or quarter of a 

 mniute, a pendulum beating feconds (or a weight fufpended by a firing of 39,2 Englifli 

 inches) may be ufed. The ofcillations are to be counted from the moment of the agi- 

 tation, and at the inftant of the thirtieth or fifteenth ofcillation, the finger being with- 

 drawn from tjie mouth of the fyphon, fuffers the water to flow. The fievc may be 

 ufed for coarfer emery. 



of metal it is well known that the figure does not continue flfait unlefs particular care be taken not only to avoid 

 partial prelTurc and expanfion, but llkcwife to fhift them with refpeft to each other, To th^t the pieces may alter- 

 nately be uppermofl. Thefe and other precautions which are fiiflRciemly known to thofe who make metallic 

 fptculums are alfo ncceflary to be attended to in the confbuQion of levels.— N. 



T. 



