562 SCIENCE PROGRESS 



If we now replace our measures of length and time by those 

 made by an observer fixed at the centre to whom we are in 

 relative motion (using the relativity equations which hold at 

 the moment), we get the equation of the orbital element for 

 this observer. The integration does not yield the well-known 

 equation of the ellipse in polar co-ordinates, viz. : 



l/r — i + e cos 9 

 but 



l/r = i -f- e cos ( i — n)6 

 where n = i27r 2 a 2 /c 2 T 2 (i — e 2 ) 



[c is velocity of light ; a, T, e semi-axis, period and eccen- 

 tricity of the approximate ellipse.] 



The smallest value of r, viz. //(i + e,), occurs therefore not 

 at 6 = o, 27r, 477-, etc., but at 6 = o, 277-/(1 —n), 47r/( I ~ n )> 

 etc. ; i.e. we have a progressive displacement of the perihelion 

 by an amount practically equal to n.2ir each revolution. The 

 calculated amount for Mercury by this formula is 42" per 

 century, which just accounts for the well-known Leverrier 

 discrepancy. 



Einstein, using the " principle of equivalence," also postu- 

 lates that to the moving observers a ray of light in vacuo also 

 has the simplest path in their neighbourhood, viz. straight, 

 and shows by the same transformation that it is curved in the 

 fixed frame of reference, a result confirmed qualitatively and 

 quantitatively by the recent Eclipse expedition. 



There still remains the third crucial test concerning the 

 displacement of the solar spectral lines toward the red by an 

 amount -008 Angstrom units. Hitherto it has remained un- 

 confirmed ; but it is only fair to add that, at the moment of 

 writing this article, a short note has appeared in Nature to 

 the effect that Einstein, in a private communication to the 

 correspondent of Nature, claims that two physicists working 

 at Bonn have detected it with certainty. No doubt the work 

 will shortly be published, and, if possible, reference to it will 

 be made in these notes in the next issue of Science Progress. 



PHYSICAL CHEMISTRY. By Prof. W. C. McC. Lewis, M.A., D.Sc, 

 University, Liverpool. 



Transition of Dry Ammonium Chloride. — The substance, 

 ammonium chloride, has had a unique interest for chemists 

 for the past ten years or more, owing to the anomalous behaviour 

 of its vapour in respect of vapour pressure. Baker showed 

 many years ago that thoroughly dried chemical substances 



