3G6 Prof. H. E, Armstrong on Low-Temperature Research 



in charcoal, 524 calories at 20° absolute. As at 78° abs. the molecular 

 latent heat of absorption of hydrogen in charcoal is 2005 calories, a 

 value about four times as great as that determined at about one-fourth 

 the temperature, it may be inferred that the molecular latent heat of 

 absorption of helium at its boihng-point, supposing this to be about 

 5° absolute, would be about one-fourth the value observed at 18°, 

 viz. ^^ = 120 calories. As the amount of heat given out in the 

 liquefaction of a gramme molecular proportion of hydrogen at its 

 boiling-point is about half as great as that given out when it is 

 absorbed by charcoal at the same temperature, by analogy it may be 

 inferred that the molecular heat of liquefaction of helium at its 

 boiling-point would be about ?4^ = 60 calories. Knowing the latent 

 heat, the boiling-point, and the fluid density from the helium charcoal 

 absorption experiments, assuming by analogy a similar behaviour to 

 that of other gaseous elements, all the data are available to calculate 

 the vapour pressures of liquid helium.* 



As bearing on the properties of the hydrogen and helium molecules, 

 these results are of extraordinary significance. It is clear that when 

 cooled to a temperature at which their motility is so much reduced that 

 they are no longer indifferent to other molecules, they are possessed 

 of powers of attraction which are by no means inconsiderable. 



Metallic Vacuum Vessels. 



One great advantage attaching to the use of charcoal is that it 

 has rendered possible the maintenance of a very high vacuum during 

 any required period of time. In the pre-charcoal period, this was 

 impossible, owing to the leakage of gas into the exhausted vessel 

 either as a consequence of the mechanical imperfection of the glass 

 vessel or because of the existence of air imprisoned in bubbles or 

 tubules within the glass : such leakage may now be counteracted by 

 means of charcoal. 



Metallic vacuum vessels could not be made formerly for a similar 

 reason, the gas occluded within the metal escaping gradually and 

 spoiling the vacuum in the vessel. By enclosing a quantity of char- 

 coal in a globular space A (Fig. 8), so that it is cooled by the liquid 

 air in the inner vessel, this difficulty is entirely obviated : such me- 

 tallic vessels are now made of nickel, brass or copper, provided with 

 necks made of an alloy of low conducting power. When properly 

 constructed, these vessels are as effective as chemically silvered glass 

 vacuum vessels ; as they are not fragile like the glass vessels, it is to 

 be expected that they will be of the greatest service in future work 

 with liquefied gases. 



The manufacture of Dewar vacuum vessels is now a German 



* The following formula gives an approximation to the vapour pressure in 

 mm. ; T being the absolute temperature log : Pjj^ = 5 • 324 — 11/T. A similar 

 formula for liquid hydrogen gives log^P^ = 5^778 — 54-6/T. 



