October 22, 1908] 



NATURE 



635 



very suggestive; it is to be regretted that one of the 

 supporters has gone; with it in position and the quoit 

 removed we have a great similarity to the leading 

 feature in the Aberdeen circles, namely, a recumbent 

 stone between two uprights. The similarity to a cove 

 is also obvious. 



Norman Lockver. 



HELIUM. 



A LITTLE more than ten years ago this remarkable 

 element was only known to astronomers through 

 the medium of the spectroscope. Now it is not only 

 to be found in all laboratories, but appears to occur 

 in almost all constituents of the earth's crust and in 

 amounts proportional to their radio-activit}', except in 

 beryl (Strutt). In some cases it occurs in quanti- 

 ties far from minute, as in certain minerals, particu- 

 larly cleveite and monazite, where the number of 

 litres of gas obtained is comparable with the number 

 of kilograms of mineral treated. Again, it con- 

 stitutes more than 5 per cent, of the gases evolved from 

 some mineral springs, as at Maizieres, and i'84 per 

 cent, of the vast supplies of natural gas at Dexter, 

 Kansas, while it occurs everywhere as four parts in 

 a million of the atmosphere (Ramsay). Its mode of 

 occurrence and origin are too complex and still too 

 uncertain to be treated adequately here, but it is 

 apparently not liberated from minerals by grinding 

 alone to an impalpable powder (Moss), and it certainly 

 permeates quartz at temperatures above 220° C, and 

 with a velocity rising with the temperature (Jaquerod 

 and Perrot). Moreover, it appears to be frequently 

 produced in the gradual breaking down of the 

 uranium molecule and the various radio-active trans- 

 formations of this into radium and other substances. 



Helium was first known from its yellow line D3, 

 and was first detected on the earth by the same 

 characteristic (Ramsay). In nitrogen or hydrogen it 

 appears that a proportion of 10 per cent, can just be 

 detected by its spectrum (Collie and Ramsay). Very 

 shortly it was shown to be a very light, unreactive 

 gas with monatomic molecules. Hence it was taken 

 to be the lightest known member of the argon group. 

 Later determinations have shown that its density can- 

 not differ much from 2'o (Onnes), and that the value 

 of the ratio of its specific heats is i'63 (Geiger), which 

 confirms the earlier results. 



The very simple character and small mass of the 

 molecule are evident in all its properties ; thus its 

 refractivity («— i) is found to be very small, but 

 various observers differ as to whether there is disper- 

 sion in the visible spectrum or not. Recent results 

 give values of 3'478x io-^-l-7'6x io-'^/a.- (Burton), 

 and 3'47x io-^-l-8'2 X 10-'"/^^ (Cuthbertson and Met- 

 calfe), in excellent agreement for a value about one- 

 quarter of that of hydrogen and with considerable 

 dispersion. Measurements on the conductivity for 

 heat K=/i7C„ show that it is greater than for other 

 gases, and appear to indicate that / has its theoretical 

 value of 2'5. They are, however, complicated by the 

 uncertainty as to the value of »;, the viscosity. The 

 viscosity with reference to air is given as 0.96 (Ray- 

 leigh). The diffusion of helium through a porous plug 

 is faster than the simple theory would indicate (Ramsay 

 and Collie), and this, together with the want of con- 

 formity in effusion results, may be partially due to its 

 very low inversion temperature (Donnan). 



The same characters are obtained under the influence 

 of electric stimulus ; thus ionic velocities of 6'3 1 cm. /sec. 

 for negative and 5'og for positive a rays (Frank and 

 Pohl) are larger values than those found for any gas 

 but hydrogen. In the vacuum tube the dark space 



NO. 2034, VOL. 78] 



exhibits several distinct maxima showing long free 

 paths (Aston). 



The Zeeman effect is extremely simple and regular, 

 the lines breaking up into normal triplets with dis- 

 persions (d\lx-) proportional to the fields up to 

 12,000 c.g.s. (Lohmann). Confirmatory observations 

 with measurements of ejni give values of this for D^ 

 (^ = 5876) of ii'3xio' across and I2'3 x 10° parallel to 

 the field, somewhat higher values being found for 

 two other strong lines, ^. = 6678 in the red and A = 5016 

 in the blue-green (Grey and Stewart). 



Gaseous helium has a small negative magnetic sus- 

 ceptibility of o'ooi75, less than argon (Tiinzler), while 

 its dielectric cohesion is the lowest known. All mon- 

 atomic gases have low values, argon being 39 where 

 hydrogen is 205, but helium is i8'3, so that o'oo5 per 

 cent, of the diatomic gas can be clearly detected 

 (Bouty). 



It would seem as if this property might be a certain 

 and easily applied method of checking the freedom of 

 helium from hydrogen. Other methods of testing its 

 purity are determinations of density and spectrum ana- 

 lysis. The former is extremely accurate when great 

 precautions are taken, but does not easily give an 

 accuracy of more than 005 per cent. ; however, with 

 the spectroscope it seems that o'ooS per cent, or less 

 of hydrogen can be seen (Onnes). 



It is, however, in its character of the most perfect 

 known g"as that helium has the most obvious useful- 

 ness, and this in two directions. At all temperatures 

 below 100° C. a constant-volume helium thermometer 

 is the most convenient and accurate known because 

 the corrections are very small and regular. Deter- 

 minations of the mean pressure coefficient from o°'o C. 

 to ioo°'o C. gave values of o'oo36624i and o'oo36627o 

 for a normal thermometer (760 mm. at o°'o C, 

 Travers and others). These values, when corrected 

 and re-calculated to the international scale (1000 mm. 

 at o°'o C), appear as o'oo366i6 and o'oo366i3, the 

 former of \vhich agrees exactly with a direct deter- 

 mination at this pressure (Onnes). To obtain these 

 corrections use has been made of isotherms observed at 

 o°'o C, 20°'o C, and 100° C, and the corrections of the 

 helium thermometer to the absolute scale are deduced 

 from isotherms at the values given. They are — o°'oo6 

 at-io3°'57 C, -f-o°'oo2 at -i82°-75 C, and +o°'oi at 

 — 2i6°'56 C, while in later measurements of the boil- 

 ing point -l-o°'2 is assumed at -268°'6 C. = 4°'5 K., if 

 absolute zero = —273°' 10 C. (Onnes). The isotherms 

 indicate that there is no minimum until about 

 -253^ C, so that the Boyle point, where {ii{pv)l(/p)t = o, 

 appears to lie at about this temperature, and hence 

 the Joule-Kelvin inversion temperature, for small 

 pressure differences, will lie at about twice this, or 

 40° K. The isotherm for — 258°'82 exhibits a distinct 

 minimum at about 10 atmospheres, as can be seen by 

 plotting the following values for pv against p; p — o, 

 fD=o'o5222; /' = 4o'oi2, ^I' = o'o6i5o; p = ^6'222, 

 /)7; = o'o65S9; p = S3'3^6, ^11 = 0-07063; ^ = s8'797. 

 />7) = 0-0753 1. 



The value of the critical temperature, which had 

 been variously given from 8° K. (Dewar) to 2° K. 

 (Olszewski), and about 1° K. (Onnes and Keesom), was 

 settled by these isotherms as not greater than 5°-3 K., 

 and later observations of the liquid fixed it as little 

 above 5° K. with a correspondingly low critical pres- 

 sure of 2-3 atmospheres (Onnes). Liquid helium boil- 

 ing normally at 4°-5 K. is very mobile, with an 

 extremely small apparent surface tension and a density 

 of 0-15, and is only eleven times denser than the 

 vapour above it (Onnes). 



In mixtures of helium and hydrogen the gaseous 

 helium sinks in the liquid hydrogen at about 40 

 atmospheres (Onnes), which opens up a wide field of 



