1896.] Helium, a Gaseous Constituent of certain Minerals. 325 



that moonlight is 44 times brighter than starlight when unabsorbed 

 by more than 1 atmosphere, and if uniformly distributed, though, for 

 illumination of a horizontal screen, it is 175 times brighter so far as 

 photographic action is concerned. If we take fche visual quality of the 

 two lights to be the same, these figures should bear the same propor- 

 tion for visual observation. If moonlight be O'Ol candle at 1 foot 

 distance, starlight will be 0'000057 candle at the same distance, that 

 is, the visual value of one candle at nearly 132 feet distant from a 

 screen. With an intensity of about 6/1000000 of candle placed at 

 1 foot from a screen, or about 10 times less illumination thau the 

 above, the screen would be invisible. It follows that the actual 

 illumination given by starlight will be less than that stated. 



Addendum. March 25, 1896. 



I ought to have drawn attention to the fact that though the above 

 comparison of moonlight with starlight was taken from actual observa- 

 tions, it would not have been unfair to have deduced the value of 

 moonlight as observed at Wimbledon with the moon in the zenith. 

 From the observations made and recorded in my paper on the 

 " Transmission of Sunlight through the Earth's Atmosphere," the 

 coefficient of absorption ft for the rays affecting the bromo-iodide of 

 silver can be shown to be 340, under the very favourable circum- 

 stances under which the exposures were given. As the rays of the 

 moon had to traverse 1*45 atmosphere, and then showed a photo- 

 graphic illuminating power of 0*266 S.C. ; had they only had to 

 traverse a thickness of 1 atmosphere, this number would have been 

 0*308 S.C. This last value would have been equivalent to a visual 

 estimation of moonlight of closely 0'012 S.C. at 1 foot. Starlight 

 would have then been rather more than 200 times less bright than the 

 light of the full moon. 



II. 44 Helium, a Gaseous Constituent of certain Minerals. 

 Part II — Density." By William Ramsay, F.R.S., Professor 

 of Chemistry in University College, London. Received 

 March 12, 1896. 



§ 1. In the original notice of this gas (' Proc. Roy. Soc.,' vol. 58, 

 p. 81), it was stated that the gas obtained from cleveite contained 

 some, but not much, nitrogen, and no hydrogen. I have since pre- 

 pared samples from broggerite, samarskite, and fergusonite, and I 

 tmd that in all cases the gas evolved on heating the mineral in a 

 vacuum is rich in hydrogen ; the amount of nitrogen is in all cases 



