450 



Mr. H. W. L. Absalom on the Ultra-violet 



Temp. 



°C. 



Compressibilty, 



/3X10 6 , 



at from 50-100 atmos. 



(V-b), 

 in c.c. 



in atmos. 







20 

 40 

 60 



80 

 100 



133 

 158 

 203 

 252 

 315 

 395 



2-98 

 3-79 

 5-25 

 689 

 913 

 1200 



7520 

 6400 

 4940 

 3970 

 3180 

 2540 



magnitude of a few thousand atmospheres; whilst, as 

 should also be the case, the values for the free space are 

 relatively small as compared with the volumes of the 

 molecules themselves. 



XLII. Studies of the Ultra-violet Transparency of Certain 

 Coloured Media. By H. W. L. Absalom, B.Sc, A.R.C.8.* 



I^HE present investigation originated in an attempt to 

 find some colouring matter which, while opaque in the 

 yellow region of the spectrum, should transmit as far as 

 possible into the ultra-violet. Such a material would be of 

 great use in developing certain investigations on the fluor- 

 escence of sodium vapour which have been made in this 

 laboratory (see Strutt, Proc. Roy. Soc. Nov. 1915). 



With this object a number of little-known colouring 

 matters were examined, and the subject was afterwards 

 pursued for points of interest it was found to possess in itself. 



The coloured metallic salts and the aniline dyes have 

 already been well explored (see Uhler and Wood, 'Atlas of 

 Absorption Spectra '), but few of them possess much ultra- 

 violet transparency. Attention was turned therefore to the 

 blue rock-salt of Stassfurt, believed to owe its colour to 

 colloidal sodium. This was found to be very transparent, and 

 the observation led on to investigation of other naturally 

 coloured minerals and precious stones as well as to various 

 preparations of colloidal metals, such as the solutions of 

 alkali metals in anhydrous liquid ammonia. 



I. Gems and Minerals. 



The arc between copper poles was employed as the source 



of radiation. By means of a small quartz spectrograph, 



photographs were obtained of the portion of the spectrum 



between \5000 and X2250. The specimen under exami- 



* Communicated by the Author. 



