400 Prof. H. E. Armstrong on Loiv-Teniperatare E^searcli 



"In the execution I have availed myself of different means which 

 Dewar has taught us to use. I have set forth the great importance of 

 his work in the region of low temperatures in general elsewhere ; here, 

 however, I gladly avail myself of the opportunity of pointing out that 

 his ingenious discoveries, the use of silvered vacuum glasses, the 

 liquefaction of hydrogen, the absorption of gases in charcoal at low 

 temperatures, together with the theory of Van der Waals, have had an 

 important share in the liquefaction of helium." 



Professor Onnes used helium extracted from Monazite sand, 

 from which the thorium now used so largely in making the mantles 

 for incandescent lighting is prepared. He operated with a quantity 

 of 200 litres (160 litres being held in reserve) but it was necessary 

 to pass this through the condensing circuit twenty times before lique- 

 faction was observed. As showing the magnitude of his operations 

 it may be mentioned that, at the commencement of the experiment, 

 at 5.45 a.m., 75 litres of liquid air were available and that at 1.30 

 p.m. 20 litres of liquid hydrogen were ready for the -final cooling 

 operation. Nothing had been observed when the last flask of liquid 

 hydrogen was connected with the apparatus : liquid helium was just 

 perceived at 7.30 p.m. 



In his Presidential Address to the British Association in 1902, 

 Sir James Dewar gave a forecast of the properties of liquid helium 

 based on his studies of the properties of the gas and the application 

 of Yan der Waal's doctrine of corresponding states, a generalisation 

 which has played a most important a part in his experiments as well 

 as in those of Professor Onnes. 



The boiling-point was estimated to be about 5° absolute ; he in- 

 ferred that the liquid would be twice as dense as liquid hydrogen (viz. 

 • 14), that it would possess a very feeble surface-tension and would 

 be only seventeen times as dense as its vapour ; also that it would be 

 quite exceptional in its optical properties and very difficult to see. 



Professor Onnes, in point of fact, had difficulty at first in 

 realising that he had succeeded in effecting his purpose : the liquid 

 looked, he says, at if it was almost at its critical temperature — in fact, 

 he speaks of the peculiar appearance of the helium as being best 

 compared with that of a meniscus of carbon dioxide in a Cagniard 

 de la Tour tube. It has exceedingly slight capillarity. The boiling- 

 point is at most 4*5° absolute. Liquid helium has a very low 

 density, viz. 0*15, the ratio of the density of the vapour to that of 

 the liquid being about 11 to 1. 



The liquefaction of helium has brought us apparently to within 

 about 3° of the absolute zero of temperature — a result nothing short 

 of marvellous. 



The history of Cold and of the Absolute Zero was discussed very 

 fully by Sir James Dewar in his British Association Address. 



He pointed out that the production of cold occupied Bacon's 



