1896.] on New Besearches on Liquid Air. 135 



of mercury is left in the vessel, the influx of heat is diminished to 

 one-sixth part of the amount entering without the metallic coating. 

 The total effect of the high vacuum and silvering is to reduce the 

 ingoing heat to one-thirtieth part, or, roughly, 3J per cent.. Vessels 

 constructed with three dry air spaces only reduced the influx of heat 

 to 35 per cent. An ordinary mercury vacuum vessel is therefore ten 

 times more economical for storing liquid air, apart from considerations 

 of manij)ulation, than a triple annular spaced air vessel. It has been 

 suggested that the metallic coating of mercury does no good, because 

 Pictet has found that all kinds of matter become transparent to heat 

 at low temperatures. The results above mentioned dispose of this 

 assumption, and direct experiment proves that no increase in the 

 transparency of glass to thermal radiation is effected by cooling it to 

 the boiling point of air.* 



An ocular demonstration of the correctness of the above state- 

 ments can easily be shown by mounting on the same stem three 

 similar double-walled test tubes, two of which have been simul- 

 taneously exhausted and sealed off from the air pump together, while 

 the third is left full of air. One of the vacuum test tubes is conted 

 with silver in the interior. The apparatus is shown in Fig. 2. A has 

 the annular space filled with air ; B and C are exhausted, C being 

 coated with silver. On filling liquid ethylene to the same height into 

 each vessel, and inserting corks with similar gas jets and igniting the 

 escaping gas, the rehitive volumes of the flames is roughly proj^or 

 tional to the influx of heat, and resembles what is shown in the drawing. 

 It is satisfactory to have independent corroboration of the advantages 

 of the use of vacuum vessels, and this may be found in a paper by 

 Professor Kamerlingh Onnes, of Leiden, communicated to the Am- 

 sterdam Academy of Sciences, 1896, entitled ' Kemarks on the Lique- 

 faction of Hydrogen, on Thermodynamical Similarity, and in the Use 

 of Vacuum Vessels,' in which he says: — "In the same degree as it 

 becomes of more importance to effectuate adiabatic processes at very 

 low temperatures, the importance of the vacuum vessels of Dewar will 

 increase. It seems to me that they are the most important addition 

 since 1883 to the appliances for low temperature research." ..." It 

 is a rejoicing prospect that practical engineers will doubtless feel the 

 want of such non-conducting mantles. For as soon as this stage is 



* At a meeting of the French Academy in 1895 a paper by M. Solvay of 

 Brussels was read, in which my 1892 device of vacuum vessels was attributed to 

 M. Cailletet, and tacitly accepted by him ! In 1875 I had already used a highly 

 exhaustive vessel, of similar shape to the vacuous test tube, in calorimetric ex- 

 periments. See paper on '* The Physical Constants of Hydrogsniura," Trans. 

 Koy. Soc. Ed. vol. xxvii. Even as late as April 1896, Professor Tilden, D.Sc. 

 F.R.S, of the Koyal College of Science, in a paper entitled " L'Appareil du 

 Dr. Hampson pour la Liquefaction de I'air et des gas," communicated to the 

 ' Revue Ge'ne'rale des Sciences,' thought proper to write as follows : " Un manchon 

 de verre, dans lequel on a fait le vide (manchon semblable a ceux de'crits par 

 Cailletet ou Dewar)." Where did Professor Tilden find Cailletet's description 

 of a vacuum vessel? This is not the only statement in the paper requiring 

 correction. 



