November 4, 1922] 



NA TURE 



619 



the ordinary liquid air process of producing oxygen 

 does not give gas of sufficient purity. 



Prof. Onnes's paper contains an account of his 

 attempts at producing solid helium and, of course, 

 the attainment of the lowest temperature. His 

 original experiments with helium date back to 

 fifteen years ago, when he found that liquid helium 

 boiling under the lowest pressure he could produce 

 (about 2-2 mm.) did not solidify. The temperature 

 was estimated as 1-15° absolute. In 1920, Onnes 

 determined to make a fresh attempt, using the best 

 pumps available for reducing the pressure. Batteries 

 of Langmuir condensation pumps were constructed, 

 twelve of glass and six of iron, all working in parallel 

 and delivering into two Burckhardt vacuum pumps 

 connected in series with a Siemens oil pump. The 

 largest Burckhardt pump was capable of dealing 

 with 360 cubic metres of gas per hour. A diagram 

 of the experimental arrangements is shown in Fig. 1. 



The complete battery of pumps was capable of 

 removing one litre (N.T.P.) of gas per hour under a 

 suction pressure of 0-005 mm., but owing to the gas 

 friction in the apparatus, the actual pressure produced 

 at the surface of the helium was only 0-012 to 0-014 

 mm. Under these conditions the lowest temperature 

 attained was 0-82° absolute. Even then helium did 

 not solidify. 



Not the least difficult part of these investiga- 

 tions is the measurement of temperature. The 

 actual temperature of the liquid is obtained by 

 calculations based upon the general equation of 

 Van der Waal and extrapolating the temperature 

 vapour pressure relationship for helium. The form 

 of the extrapolated curve was compared with those 

 obtained for various other elements as the line shows 

 a decided curvature at normal temperatures. At the 

 meeting Prof. Porter discussed the theoretical basis 

 of this method of extrapolation and the possible error. 



The two papers from the Leyden laboratory should 

 prove of material assistance to the student interested 

 in the technique of low-temperature investigations. 



The industrial application of the liquefaction 

 processes was dealt with by three speakers. Mr. 

 K. S. Murray gave a general account of the processes 

 employed by the British Oxygen Co. It was interest- 

 ing to hear that the efficiency of the liquefaction 

 process is not appreciably greater than that of the 

 old barium oxide process using the reversible pressure 

 reaction. The advantage of the liquefaction process 

 is that it produces a purer gas. Figures for the cost 

 of producing oxygen were given, as well as technical 

 descriptions of the various types of rectification 

 apparatus. 



The second paper was sent by M. Claude, and in it 

 was described a plant for the separation of hydrogen 

 from water gas by a liquefaction process. The 

 method can be utilised when the gas, such as that 

 from coke ovens, is too impure to permit of the use 

 of the catalytic reaction depending upon the con- 



version of carbon monoxide to carbon dioxide. The 

 plant described is used for supplying hydrogen to a 

 synthetic ammonia apparatus producing 5 tons of 

 ammonia per day. An interesting feature of the 

 apparatus is the introduction of small amounts of 

 nitrogen into the system to serve as liquid nitrogen 

 lubricant in the expansion engine. 



In the third paper, Mr. E. A. Griffiths gave an 

 account of the use of oxygen in breathing apparatus 

 for airmen, and also of the plants for manufacturing 

 liquid oxygen for this purpose. The chief difficulty 

 in the use of cold liquefied oxygen is that of storage 

 and transport. The mechanism of the metal vacuum 

 vessel, which is the only practicable solution of the 

 problem, was briefly dealt with. The manufacture 

 of these vessels is a simple matter, and the thermal 

 losses in properly constructed vessels is 12 per cent, 

 of the liquid oxygen content per day for a flask of 

 two litres capacity, while for a twenty-five-litre flask 

 it is only 4 \ per cent. 



The vaporisers for converting the liquid oxygen 

 into gas at a rate which can be kept under control 

 were described. In view of the simplicity of these 

 devices, it is surprising that greater use is not made 

 of liquid oxygen in medical and experimental work. 



The portable plants employed for producing oxygen 

 utilise both the Claude and the Linde principles. 

 Although the theoretical efficiency of expansion 

 with external work is about three times that possible 

 with the Joule-Thomson free expansion, the actual 

 results obtained on test are not appreciably different. 

 This appears to be due to the practical limitations 

 of the expansion engine. A similar conclusion was 

 arrived at independently by Mr. Murray in the case 

 of large plants. 



The expenditure of power for the production of 

 oxygen is of the order of i\ to 3 H.P. per litre/hour : 

 the figure for the Pictet cycle, according to Crommelin, 

 is decidedly lower, being only 1-64 H.P. per litre/hour. 

 The over-all efficiency of liquefaction processes is 

 therefore extremely low and generally less than 3 

 per cent. 



The remaining papers were contributed by investi- 

 gators working under the direction of the Engineer- 

 ing Committee of the Food Investigation Board 

 (Department of Scientific and Industrial Research). 



Dr. Ezer Griffiths dealt with the determination 

 of thermometric lag in various types of thermometers 

 and with some new materials for thermal insulation 

 at low temperatures. 



Prof. C. F. Jenkins gave a summary of his work 

 on the thermal properties of ethyl chloride. His 

 research on this substance is an extension of his 

 previous work on carbon dioxide. The data which 

 he has now obtained should be of considerable value 

 to the refrigerating engineer, for ethyl chloride has 

 many advantages over ammonia and carbon dioxide 

 for use in small refrigerating plants. 



E. A. Griffiths. 



Propagation of the 



T T has frequently been noted that on the occasion 

 -^ of great explosions there are curious anomalies 

 in the propagation of the sound. Usually there is 

 a normal zone of audibility in the immediate neigh- 

 bourhood of the explosion, beyond this a zone of 

 silence, where the sound is not heard, and again out- 

 side the zone of silence a second zone of audibility. 

 It is remarkable that while an observer at say 50 

 miles away may not hear an explosion, an observer 

 at 80 miles may hear it distinctly. 



These abnormalities are closely connected with 

 the meteorological conditions, though the detailed 



NO. 2766, VOL. I IO] 



Sound of Explosions. 



relationship between them is not known. One 

 theory is that the wind lifts the sound over an area and 

 brings it down again many miles away. Another 

 theory ascribes the zone of silence to the effect of 

 the distribution of wind and temperature at high 

 altitudes. The theoretical development of the 

 problem is extremely complex, and so it was decided 

 to make an experimental study of the meteorological 

 conditions along with detailed observations of the 

 extent of the zone of silence in the hope of elucidating 

 the relationship between them. 



The International Commission for the Investigation 



