180 DR. TRAVERS AND OTHERS ON THE MEASUREMENT OF TEMPERATURE. 
tar the most perfect iliermometric substance known to us. Possibly, even at the lowest 
temperature that Ave have arriA^ecl at, the temperatures measured on the constant- 
Amlume helium scale are not far from absolute measurements. 
Turniug to the cpiestion of lirpiefcing lielium, it AAmuld of course be possible to 
coniju’ess large quantities of the gas, and aU'AAving it to expand adiabatically to at 
least produce a mist of helium. As the comiAression could only be effected in A'essels 
Avith AA'alls of considerable thickness, it would probably be impossible to produce anA* 
quantity of liquid in this Ava3\ With regard to the application of the regenerative 
process, which can so easily be employed in the case of air or of hydrogen, to the 
liquefaction of helium, we can say nothing; for apart from the experimental 
difficulties involved in the problem, we do not yet knovr wdiether helium becomes 
heated or coded Avhen allowed to expand freely at high or at Ioav temperatures. 
Fi'om this point ol vieAv, and for the correction of gas thermometers, the Joule- 
Thomson effect for h'dium should be determined over a Avide range of temperature. 
In conclusion, Ave Avish to express our thanks to the Government-Grant Committee 
of the lloyal Society for the assistance they have given us in carrying out this 
iiiA-estivation. 
O 
