﻿950 The Protection of Brass Weights. 



crucible is then covered with its lid, and the whole heated 

 with a Bunsen flame. (It is convenient to adjust the flame 

 so that it extends about half-way up the crucible.) During 

 the heating the weight is frequently viewed, and when it has 

 assumed a golden tint the process is complete ; the flame is 

 then removed, and the closed crucible allowed to cool. 

 Finally, the weight is rubbed with an old silk handkerchief 

 and then adjusted. Treated thus, the weight presents the 

 appearance of well-polished and lacquered brass. The 

 protecting film being tough, is not readily defaced, and, 

 if necessary, the polishing may without risk be prolonged ; 

 but when the initial operations are correctly performed, 

 nothing beyond a light and brief rubbing is required. 



In dealing with a number of weights, great economy both 

 in time and labour may be effected by substituting for the 

 crucible a vitreosil muffle. The muffle is fitted with a rect- 

 angular tray, which carries the bits of porcelain, placed in 

 groups of 3, for supporting the various weights. The 

 weights having been arranged, the tray with its charge is 

 introduced into the muffle, which is then closed, suitably 

 heated, and subsequently allowed to cool ; the weights are 

 then ready for polishing and adjusting. It may be observed 

 that as the supporting fragments are pointed, they but 

 barely engage the surfaces in contact with them ; hence 

 the finished protective film is practically complete. 



Some experiments were made to determine the average 

 mass of the protecting film ; the results obtained with two 

 weights, the one of 50 and the other of 20 grins., may be 

 cited. The cleaned and polished weights were first accurately 

 weighed, then protected and finished as already described ; 

 finally they Mere re-weighed. The larger weight had in- 

 creased by *0020 grm. and the smaller by *0006 grm. As 

 the respective areas of the two weights were approxi- 

 mately 17 and 12 cm., we find that the average mass 

 of 1 sq. cm. of film was '00012 grm. in the one case and 

 •00005 grm. in the other. It was found that the smaller 

 value most nearly represented the weight of a normal film ; 

 the other and larger value was exceptional. 



Daubeny Laboratory, 



Magdalen College, 



Oxford. 



