January 19, 1899] 



NA TURE 



281 



When liquid hydrogen freezes air out of a sealed tube by 

 immersing the end in the liquid, it is to be inferred that no 

 measurable pressure of air ought to be left in the vessel. If we 

 apply Van der Waals's law of corresponding temperatures to the 

 case of hydrogen, the above inference is made unimpeachable. 

 An approach to some knowledge of what the tension of air 

 must be about the boiling point of hydrogen can be attained by 

 exterpolating the vapour pressure curves of oxygen and nitrogen. 

 Taking the following range of boiling point temperatures for 

 nitrogen and oxygen, viz. from the critical point to the boiling 

 point under diminished pressure, two Willard Gibbs formula; 

 were calculated, with the following results :— 



Nitrogen 



fTemp. abs 127° 78-6' 59° 



\ Pressure in mm. ... 25,900 740 26 



Nitrogen. Iogi„/ = 11-556 



■ y& ••' I Pressure in mm. 

 Oxygen. ... \oZv\P = 9"4699 



400 '02 

 T 



•898olog,„T ,„(!). 



4222 

 T 



37>592 740 7-5 



0-9843 logi,jT..(2). 



Another Gibbs formula was calculated, taking Estreicher's 

 values for the vapour pressure of liquid oxygen below its 

 boiling point, viz. : — 



/ Temp, abs 91-44" 



I Pressure in mm. 743-8 



781° 

 141-8 



62-8 



7-5 



Oxygen. 



log,„/ = 16-0670 - 524-72 _^,go24iogj„T.. (3). 



We deduce from these formula' the following vapour pressures 

 at the temperature of boiling hydrogen : — 



The results of calculation, taking the formula; for the widest 

 range of pressures, viz. (l) and (2), may probably be the surest, 

 but in any case those values must be taken as a inaximuin, 

 seemg they refer to the liquid state, while both oxygen and 

 nitrogen, at the temperature of 35° absolute, are hard solids, 

 and must therefore have dropped to lower tensions than that of 

 the exterpolated liquid vapour pressure curves. It is curious to 

 note that at this low temperature the theoretical ratio of the 

 tensions of nitrogen and oxygen is as 20 to i. Direct measure- 

 ments of the vapour pressure of nitrogen at the melting point, 

 or 60° absolute, gave the value of 26 mm., and a ratio of the 

 tensions of nitrogen to oxygen of 6 to i, whereas from the curves 

 the value ought to be 6-7 to i. Olszewski gives the tension of 

 nitrogen at - 214° as 60 mm., and as at this temperature the 

 oxygen tension is 3-8 mm., the ratio of the saturated pressures 

 of the two gases at the melting point of nitrogen would be as 

 16 to I, which is far too high. Probably the oxygen value will 

 be nearest the truth, seeing it has the lowest melting point. The 

 tension is about a ten millionth of an atmosphere. In the case 

 of nitrogen, the maximum theoretical pressure would be one 

 five hundred-thousandth of an atmosphere. It is safe to infer 

 that the vacuum left after liquefying the air out of a vessel by 

 means of liquid hydrogen cannot exceed the millionth part of 

 the atmospheric pressure, exclusive of the pressure resulting 

 from any incondensable material other than nitrogen and oxygen. 

 This is just about the pres.sure of the vapour of mercury at the 

 ordinary temperature in the Torricellian vacuum, so that as good 

 an exhaustion ought to result as can be got by boiling out a 

 space with mercury. There is another way in which the 

 question may be put. Assuming the molecular latent heats are 

 approximately proportioned to tlie absolute boiling points, then 

 we can, from a comparison with the oxygen value, deduce that 

 of hydrogen, and thereby get the constants in a two term 

 formula for the vapour pressures. For pressures below an 

 atmosphere, the following approximate formulae were deduced : — 



O.xygen log/ = 7-205S 



392-6^ 

 T 



(4). 



Hydrogen ,„ log/ = 7-2428- '^i:? , 



From these expressions it follows that at its boiling point, or 

 35° absolute, hydrogen has 7/852000 times the pressure of 

 oxygen, or the latter pressure is about the eight millionth of an 

 atmosphere, A similar formula, calculated from the critical 

 and boiling point data, gives substantially the same order of 

 quantities. Formulae (4) for oxygen tensions must be fairly 

 accurate, seeing it gives a theoretical latent heat of about 56 

 units per gram of liquid evaporating at the boiling point, whereas 

 direct determinations result in 55 units. To test this inference, 

 the following plan of experimenting was adopted : — Ordinary 

 shaped vacuum tubes, like A, B, used for the spectroscopic 

 examination of gases, with and without electrodes (Figs. I and 

 2), having a capacity ranging from 15 to 25 c.c. , had pieces of 

 quill tubing about a foot long sealed on. The tubes were con- 

 tracted at D to about 1 mm., so that they could be sealed off 

 with rapidity. The end c sometimes terminated in a small bulb 

 (Fig. 3), in order to give increased cooling surface, and, when 

 necessary, to allow many times the volume of air in A, B, to 

 enter and be condensed with the object of accumulating any 

 incondensable residuum. 



The tubes were filled with air, oxygen, and nitrogen at the 

 atmospheric pressure. The liquid hydrogen collected in the 

 vacuum vessel, immersed in another similar vessel full of liquid 

 air, being ready, the end C was dipped in the liquid for a little 

 over a minute, and the tube AB sealed off at D, so that on 

 removal from the hydrogen bath the solid air might melt and 

 distil off without generating any pressure. On attempting to 



NO. 1525, VOL. 59] 



pass the spark through vacuum tubes prepared in this manner, 

 their excellent exhaustion was revealed by great resistance to 

 the passage of the discharge, and the high phosphorescence of 

 the glass. Two tubes, kindly prepared by Sir William Crookes 

 with platinum electrodes that he had previously sparked to 

 remove gases and impurities on the glass before filling with dry 

 air, gave, when treated in the manner described, such high 

 vacua that the tubes had to be heated in order to get any spark 

 to pass. Thus it is proved that the tension of solid nitrogen 

 and oxygen at the temperature of boiling hydrogen is below the 

 millionth of an atmosphere, seeing there is less difficulty in 

 getting a discharge to pass in tubes exhausted to this extent. 

 In Older to get some definite idea of the limit of the exhaustion 

 produced, two tubes, such as have been described as suitable 

 lor the liquid hydrogen experiments, might be joined together 

 and filled with oxygen or nitrogen at atmospheric pressure, and 

 simultaneously exhausted with the mercurial pump to a small 

 fraction of an atmosphere, and then -sealed off from the pump 



