312 EEPOKT— 1880. 



A Altitude. 



Eiffielberg . . 293-2 SSTO" 



Eigi . . . 294-8 1650 



Viege . . . 295-4 660 



Diff. (Riffel-Vifege) - 22 1910 



The conditions of the foregoing experiment were obviously more 

 favourable to the coiTect determination of the influence of different 

 thicknesses of atmosphere in absorbing the extreme rays than were those 

 previously made at low levels. It would appear, then, that the increase 

 of length expressed in wave-lengths is about one-millionth of a milli- 

 metre for 900m. within the limits experimented on by M. Cornu. 



Notwithstanding the failure on the part of Dr. Miller to trace any 

 special connection between the chemical complexity of a substance and 

 its diactinic power, Mr. "W. N. Hartley, in the year 1872, having at his 

 disposal the apparatus which had been used by Dr. Miller, determined to 

 repeat, in a more complete and comprehensive manner, the experiments 

 which had been made by that investigator. He was led to this determi- 

 nation by the consideration that all the characteristic physical properties 

 of organic substances are dependent on their molecular constitution ; and 

 he inferred that if a large number of bodies of similar constitution were 

 examined, many of which would be metaraeric substances, such as the 

 ethereal salts of the organic acids and homologous series of the normal 

 alcohols and acids, evidence might be forthcoming of the influence of 

 impurities and the variations in the absorption of the invisible rays 

 caused by each increment of CH2 in the molecule. In the carrying out 

 of this research, Mr. A. K. Huntington was associated with Mr. Hartley ; 

 their joint labours ai'e recorded in the ' Phil. Trans.' part I., 1870, and in 

 the ' Proc. Roy. Society,' 1879. 



The relative absorptive power not being affected by the physical 

 condition of matter (Miller), the inconvenience of making observations on 

 equal volumes of organic substances in a state of vapour was avoided, it 

 being easy to arrive at the maximum absorption due to a molecule of a 

 substance by taking into account its specific volume in the liquid state, 

 and making the layer of liquid proportionally thick, or by dissolving the 

 substances in solvents of known transparency in the ratio of their mole- 

 cular weights. 



The method of experimenting. 



After careful trial of the methods of studying the ultra-violet rays, 

 preference was given to the photographic method. Rays which cause a 

 very indistinct eSect, or no effect at all, on a fluorescence screen, will on 

 a properly prepared photographic plate produce a satisfactory image. A 

 piece of uranium glass is extremely useful in focussing ; a strip of glass 

 coated with gelatine in the solution of which some aesculine has been 

 dissolved, answers equally well. To observe the visible and ordinarily 

 invisible rays simultaneously by reflected light, a piece of paper steeped 

 in a solution of eesculine, to which a little ammonia has been added may 

 be employed. 



In the course of the investigation it was found necessary to modify in 

 many important details the original apparatus. In order to prevent the 

 ignition of volatile liquids and to better concentrate the light on the slit, 

 the liquids under examination were placed at the back of the slit, in a 

 box forming a prolongation of the collimator tube. A means of exhausting 



