1893.] 



on Liquid Atmospheric Air. 



Fig 5. 



is convenient and more effective to cause the deposition of a mercury 

 mirror over the surface of the inner vessel (by leaving a little liquid 

 mercury in the lower part of the double-shaped flask), instead of 

 silvering as previously described. Under such conditions the mer- 

 cury instantly distils and forms a brilliant mirror all over the surface 

 of the inner vessel. The fact that mercury has a very high refrac- 

 tive index and is a bad conductor of heat are factors of importance 

 in retarding the conveyance of heat. After the mercury mirror 

 has been formed any further increase in the thickness of the film 

 can be prevented, and at the same time the vacuum improved by 

 freezing the excess of liquid mercury in the lower part of the vessel. 

 The vacuum vessels described equally retard the loss as well as the 

 gain of heat, and are admirably adapted for all kinds of calorimetric 

 observations. The future use of these vessels 

 in thermal observations will add greatly to 

 the accuracy and ease of conducting investiga- 

 tions. The double spherical form of vacuum 

 vessel is excellent for showing that the elevation 

 or depression of a given volume of air a few feet 

 causes an increase or diminution of volume, due 

 to the small change of atmospheric pressure. 

 The volume of air in the inner sphere is guarded 

 from any sudden change of temperature by the 

 surrounding highly vacuous space. This is only 

 one of the many uses to which such receivers can 

 be put. 



In making vacua, many other substances have 

 been examined along with mercury, but they have 

 not given equally satisfactory results. 



Sulphur would occur to any one as a substance 

 that might replace mercury, seeing the density 

 in the form of vapour, and also the latent heat 

 of vaporisation, are nearly identical ; and it has 

 the further advantage of being a solid at ordinary 

 temperatures. The sulphur vacua have, however, 

 so far not been an improvement, chiefly because traces of organic matter 

 are decomposed by the sulphur, giving sulphuretted hydrogen and sul- 

 phurous acid, gases which are dissolved by and remain in the sulphur. 



When the surface of such a sulphur vacuum is cooled with 

 liquid oxygen in the manner previously described, a faint crys- 

 talline deposit occurs, only it takes a much longer time to appear 

 than in the case of the mercury vacuum. If a similar vessel is 

 boiled out, using phosphorus as the volatile substance, the application 

 of liquid oxygen to the surface causes instant deposition. Thus it 

 can be proved sulphur and phosphorus distil at ordinary temperatures 

 just like mercury. 



An investigation as to the electric conductivity of metals, 

 alloys, and carbon at low temperatures has been undertaken in 



