ATTAINMENT OF VERY LOW TEMPERATURES. 21 



the coil can be reduced to below — 200° C. Meanwhile liquid air is 

 poured into the chamber B, and by opening the cock t some of it ia 

 drawn into the chamber C. 



IX. Hydrogen Liquefier with Second Regenerator Coil. 



In each of the different forms of the hydrogen liquefier which have 

 hitherto been described there exists one very obvious defect: the 

 hydrogen, after expanding at the valve E and passing through the inter- 

 stices of the regenerator coil D, is at a temperature not far from 

 — 2io°C., and in this condition it leaves the apparatus, absorbing heat 

 from the walls of the tube through which it passes on its return to the 

 compressor. 



Now if we assume that the specific heat of hydrogen at a constant 

 pressure of one atmosphere is equal to 3.41 units over the range of 

 temperature we are considering (see page 14), the heat which each 

 gram of hydrogen would absorb in returning to the normal temperature 

 would be 



3.41 X 220 = 750 units. 



As approximately 1,000 grams of hydrogen pass through the apparatus 

 per hour, this involves a loss of 750,000 heat units, the equivalent of 

 fifteen liters of liquid air. 



In order to estimate the relative advantage of employing the cold 

 hydrogen in place of solid carbonic acid as a means of cooling the 

 incoming gas, it would be necessary to determine the specific heat of 

 the compressed hydrogen over the range of temperature. The meas- 

 urements could not, however, be carried out with sufficient accuracy to 

 be of any real value without great difficulty, and, indeed, it would 

 make no appreciable difference were the specific heat of the hydrogen 

 half again as great. 



If one assumes that the relationship between the internal energy of 

 the compressed and expanded hydrogen can be calculated, as in the case 

 of air, from the formula (page — ), 



JQ=^K{Jp)Cp, 



where J^ is the heat generated or absorbed by the gas during its 

 passage through the regenerator coils of the apparatus, K the Joule- 

 Thomson Effect at which it enters the apparatus, Jp the fall of pressure 

 in the apparatus, and Cp the specific heat of the gas under a constant 

 pressure of one atmosphere, — to cool the gas compressed to 150 atmos- 

 phere from I5°C. to — 75°C. it is necessary to absorb 



90 X 341 + 0.09 X 150 X 341, 



