6o6 



NA TURE 



[April 30, 1908 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communtcations.} 



The Presence of Water Vapour in the Atmosphere of 

 Mars. 



In Febi-iiary .and March, 1908, Mr. V. M. Sliphi^r 

 succeeded in photographing the lines duo to water vapour 

 in the atmosphere of Mars. It was in the " a " band 

 that its presence stood revealed. .The detection was 

 rendered possible by the use of plates specially prepared 

 by him for the purpose — 23 Seed, bathed in a mixture of 

 pinacyanal, pinaverdol, dicyanin, water, and ammonia — 

 which enabled the spectrum to be photographed somewhat 

 beyond the " A " band. Comparison spectrograms were 

 made of the moon at approximately the same altitude on 

 the same plates, and with exposures to give a like density 

 for both. Repeated plates of the sort were taken, and 

 their consensus shows unmistakably the " a " band 

 stronger in the spectra of the planet than in those of the 

 moon. In the case of the moon, of course, we are look- 

 ing through our own atmosphere only ; in the case of the 

 planet, through its atmosphere as well. 



Previous observers — Huggins, Jansen, Vogel, Campbell 

 — had reached discordant conclusions, Huggins and Vogel 

 asserting the presence of water vapour in the atmosphere 

 of the planet, Campbell with much improved spectroscopic 

 means failing to get any indication of it. The reason of 

 this was the instrumental inability at the time these re- 

 searches were made to examine the spectrum sufficiently 

 far into the red, for it is in the " a " band that the 

 greatest absorption of water vapour occurs, and this was 

 not only beyond the possibility of photography at the time, 

 but beyond even that of visual detection. Thus Vogel 

 went no further redwards than " C," while Campbell tells 

 us in his account of his researches, in which he came to 

 a negative conclusion : — " It is impracticable to observe the 

 groups A, 7450 to 7160 and 7160 to 6S70, which are at 

 the extreme red end of the spectrum, and they will not be 

 further considered." In this omission, rendered necessary 

 by the instrumental appliances at the time, lay the failure 

 to perceive the evidence of water vapour in the spectrum 

 of the planet. For, as the following table shows, the 

 intensity of the absorption is much greater in the " a " 

 band than in the lines between it and the D lines, or even 

 in those near the D lines themselves. This is borne out 

 by examination of Mr. Slipher's plates, in which the differ- 

 ence in the " a " band is evident, the broadening of the 

 D lines just perceptible, and nothing predicable of the 

 fainter water-vapour lines. 



Relative Strength of the Water-vapour Lines in the 

 Spectrum, according to Rowland. 



Determination Determina- 

 1893- tion .895. 



Subs:ance Strenglh Wavi 



A 7604 ... Oxygen 



probably certainly 

 idemified identified 



... 120 ... 120 ... - .. 7672] 

 7594/ 



a 7165 ... Water Vapour ... 124 ... 50 ... 12S ... 1^12} 



,, ... 9 ... — ... 6 ... 7016 



„ ... 42 ... .3 .. 54 ... gl} 



B 6S67 ... Oxygen 170 ... - ... - ... ^936| 



C 6562 "1 „, . ,, 6i;72i 



'Solar, H)M^=''"^'^P°"'^- 5- 5 ■•■ 20 ... ^57^)- 



Own .4 ..-..- ... 1%} 



Wa'er Vapour ... 2 ... 2 ... 10 ... "iqv? 



0,5896 1 \ 



, D= 5S90 - „ ... 26 ... 26 ... 68 ... 5920-1 



(Solar, Na)) S884/ 



No watei vapour lines of less wave-length than 5884. 



NO. 2009, VOL. 77] 



The great dryness of Arizona was no less a factor 

 in the result. So dry was the air at times during the 

 investigation that on more than one plate the " a " band 

 is hardly to be made out in the lunar spectra, while in 

 the Martian it is unmistakable. Great dryness in the 

 climate is in other ways shown by the plates to be essential 

 to the recording of a perceptible difference between the 

 water-vapour lines due to Mars and the earth and those 

 due to the earth alone. For examination of the oxygen 

 bands. A, B, and a, in the two spectra reveals no per- 

 ceptible difference between them, and yet the presence of 

 water vapour in the spectrum of Mars is strong pre- 

 sumptive evidence that free oxygen exists in its atmosphere 

 as well, since it is the heavier of the two. 



Perciv.al Lowell. 



Lowell Observatory, Flagstaff. 



The Condensation of Helium. 



I HAVE just read with great interest of Prof. Onnes's 

 experiments with helium, and as one who has carried out 

 investigations at very low temperatures, and to a certain 

 extent in the same direction, I must congratulate him on 

 having overcome difficulties of no ordinary nature. At 

 the time of my departure for India I was engaged in an 

 attempt to measure temperatures below the temperature of 

 solid hydrogen, with the ultimate object of determining 

 thermal constants for helium, but the work was broken off 

 when I left Bristol, 



and it is not likely that \ TO PUMP^ ^TO MANO - 

 I shall be able to re- ' - |« | METER. 



sume it for some time. 

 However, it is possible 

 that brief information 

 as to the method I in- 

 tended to employ may 

 be of use to others. 



The measurement of 

 low temperatures by 

 any means other than 

 by the gas thenno- 

 meter appeared to me 

 to be mere waste of 

 time, and I decided to 

 employ in these experi- 

 ments a constant 

 volume helium thermo- 

 meter. Following the 

 method of Olszewski, 

 I proposed to compress 

 helium to about 100 

 atmospheres in a vessel 

 cooled in solid hydro- 

 gen, and containing 

 the thermometer, and 

 to measure the fall of 

 temperature on ex- 

 panding the gas. The 

 only obvious diflficully 

 lay in the construction 

 of the apparatus. 



The apparatus shown 

 about natural size in 

 the figure was made 

 for me by Messrs. A. 

 Hilger. The outer 

 vessel was of thin 

 steel, and had a 

 capacity of about 

 40 c.c. Within were 

 three concentric test- 

 tubes, made as light as 

 possible, and separated one from the other and from the 

 steel vessel with fragments of cork. In the centre is a very 

 light bulb of steel, to which was soldered a capillary steel 

 tube such as is used for hypodermic needles. This bulb was 

 intended to serve as a thermometer, the steel tube com- 

 municating with the manomctric portion of one of the 

 thermometers which I employed in the measurements of the 

 temperatures of liquid and solid hydrogen (Phil. Trans., 

 cc. A, 105, 1902). A steel tube connected the steel vessel 



STEEL THER- 

 MOMETER 

 BULB. 



