374 Prof. Joly and Mr. Poole : Attempt to determine 



The method of procedure in conducting an experiment 

 was as follows : — The steel tube was first carefully cleaned 

 and then filled to the requisite height with lead which was 

 melted in a small porcelain crucible. The lead used was 

 pure lead obtained from Johnson, Matthey & Co. The 

 small lid was placed on the steel tube, and the three other 

 carriers of the centrifuge carefully balanced against the one 

 con faming the steel tube and lead. The carriers were then 

 replaced in the centrifuge and the lead melted by turning on 

 the heating current for about ten minutes, after which time 

 the motor driving the centrifuge was started, and usually the 

 centrifuge was kept running for about an hour before being- 

 stopped. It was found impossible to run the centrifuge for 

 longer as the motor was inclined to overheat after this 

 period. When the centrifuge was stopped the lead was 

 removed, while still liquid, in six lots by means of glass 

 pipette arrangement The density of the top and bottom 

 portions of the lead was then determined. 



The density of the lead was determined by casting small 

 spherical bullets from it in an iron bullet-mould. These 

 bullets were weighed first in air and then suspended in 

 methylene iodide. Methylene iodide is especially suitable 

 for this purpose, both on account of its high specific gravity 

 (about 3*3) and also its small surface-tension. Fortunately 

 it only attacked the lead very slightly, producing a very 

 slight tarnish on the surface. Some trouble was experienced 

 at first in obtaining sound castings from the lead, but it was 

 found that by allowing the mould to cool slowly from the 

 bottom upwards, bullets free from all cavities could be 

 obtained. It is essential that the conditions under which 

 the bullets are cast should be as nearly identical as possible, 

 as unfortunately the density of lead is largely affected by 

 the heat treatment it receives. However, the results show 

 that this source of error was eliminated. The balance used 

 was sensiiiveto yVmgrm., and the weights were standardized 

 against a new set by Becker & Co. which had a certificate 

 from the N.P.L. certifying them as correct to t$ mgrm. 



Summary of Me suits. 



Note : — W = weight of bullet in air, 



B = loss of weight in methylene iodide. 



w . 



Hence -^ is proportional to density. 



