172 PROCEEDINGS OF THE AMERICAN ACADEMY. 



known coefficients of expansion of the three metals. Care must be taken 

 that the bearing surface of the nut, C, on the washer, W, is so large that 

 the upward force of the steam acting on the cover does not compress the 

 brass washer, and thus allow the cover to rise. Care must also be taken, 

 — and this is very important, — that the distance from the centre of this 

 bearing surface to the axis of the bomb is less than the radius of the gold 

 packing ring ; otherwise the cover might turn on the ring while the nut 

 was being tightened, which would prevent a tight joint from being secured. 

 Various lubricants have been tried on the screw and on the top of the 

 brass washer ; but all of these, with the exception of graphite, have 

 proved to be worse than nothing : finely powdered graphite used dry is 

 entirely satisfactory, however. 



To facilitate the removal of the platinum lining, the inside of the steel 

 shell was made slightly tapering (about 0.05 mm. in 10 cm.), and the 

 little grooves left by the boring tool were carefully ground out. 



In working with the bomb it proved to be necessary to drill a number 

 of small holes through the steel shell, one of which is shown at H in 

 Fig. 1. In the present bomb there are about 75 of these (probably 

 half as many would have sufficed) well distributed over all its parts, 

 A, B, and C. These holes are 66 mm. in diameter, — so small that 

 they do not seriously weaken the shell, and that the platinum lining is 

 capable of withstanding the pressure over their areas. These holes are 

 made necessary by the fact that without them some water gets trapped 

 between the lining and the shell, owing to slight leakage or permeation 

 of the platinum itself when the bomb is first heated, the lining then being 

 not in close contact with the shell at every point ; and this water on sub- 

 sequent heating exerts, owing to its expansion in the liquid state, an 

 enormous pressure against the lining, causing little indentations in it and 

 causing some water to flow back into the bomb, whereby contamination of 

 the solution with iron is produced. The holes remedy entirely this diffi- 

 culty, which otherwise becomes aggravated on each successive heating. 



The lining of the lower part of the bomb, A, was made of a platinum- 

 iridium alloy (2 per cent iridium) 0.40 mm. thick ; but pure platinum 

 would have been preferable, owing to its greater ductility, and it will be 

 used in future work. Tiie lining was made by Baker & Company of 

 Newark, N. J. The flange, F, was originally made of the same material, 

 but the closing of the bomb compressed the platinum each time under 

 the ring so that it grew hard and thin and finally cracked at the bottom 

 of the groove. For this reason platinum-iridium alloy containing 15 per 

 cent of iridium had to be substituted for the flange. This is so hard that 



