f)10 Sir James Dewar [Jan. 17, 



19U6.* The requisite heat isolation necessary to raise the standard 

 of metallic vessels to anything like the same efficiency as those made 

 •of silvered glass, can only be attained l)y careful construction. The 

 three main drawbacks to the use of metal instead of silvered glass 

 are : (1) increased conductivity ; (2) decreased reflecting power ; (3) 

 greater difficulty in securing and maintaining the necessary very high 

 vacuum. 



1. The first objection is met by reducing the diameter and thick- 

 ness of the double neck, as this is the only bridge between the inner 

 and outer shells by which conduction of heat can take place. But 

 while the diameter and thickness are made as small as possible, the 

 neck is made as long as is consistent with rigidity and handiness. 

 By so doing not only is the conductivity loss avoided, but the con- 

 vection disturbance caused by the high temperature gradient is greatly 

 reduced. Tbe proportionate losses due to all these neck effects 

 became rapidly less as the capacity of the flasks was increased. 



2. Xo metallic container can be really efficient unless the surfaces 

 exposed in the annular vacuous space are highly polished. The final 

 buffing must not involve the use of any oily material, as this would 

 increase the emissivity, and add to the difficulties of the subsequent 

 exhausting. 



A copper surface is preferable to one of brass ; and several success- 

 ful copper containers up to 12 litres capacity were constructed. In 

 these cases, however, the neck was made of stiffer and less conducting 

 alloy. With capacities greater than 12 litres a more ■ rigid metal 

 was also desirable for the spherical shells, to withstand distortion. 

 Gilding-metal brass of good (juality is preferable to copper, despite 

 the greater emissivity of the former ; but with all large vessels it is 

 necessary to strengthen the equatorial joint, more especially of the 

 inner shell. The soldered joint must have quite 2 cm. depth of close 

 ))earing surface ail round. An additional precaution is afforded by 

 brazing not less than six circular flat lugs round the rims of each of 

 the separate halves of the shell. The lugs are on the inside of the 

 upper half, which fits into the lower half with the corresponding lugs 

 on the outside. Before soldering the joint between the two, the 

 inner and outer lugs are screwed together all round, the necessary 

 holes having been previously bored and tapped. This construction 

 was especially desirable in the larger copper vessels. 



EXHAUST OF METAL CONTAINER. 



3. As already stated, the chief objections to metallic vessels are 

 {a) the difficulty of eliminating minute pores and leaks, and (b) the 

 difficulty of removing the gases absorbed both by the metal and the 

 charcoal. The gases remaining in the pores of the charcoal are 



Proc. Roy. Inst., xviii. 349. 



