304 EEPOEX— 1880. 



M. Becquerel ; ' but. liis restolts were vitiated in consequence of his Having 

 used glass instead of quartz apparatus. 



Both Prof. Stokes and Dr. Miller carried on their experiments by 

 means of an induction coil and a Leyden jar ; the metals for the points 

 between which the spai'k passed being varied according to circumstances. 

 The rays which escaped absorption by the substance interposed in their 

 path were caused to pass through a quartz prism, and then focussed on a 

 fluorescent screen or on a photographic wet plate : the former method 

 being employed by Pi'of. Stokes,^ the latter by Dr. Miller.^ 



•Dr. Miller's experiments are comprised under the following heads : — 



(1) The ahsorption of tJie invisible rays hy transmission thmigh 

 different media. 



a. By transmission through solids. 



1). By transmission through liquids. 



c. By transmission through gases and vapours. 



(2) The ahsorption of the invisible rays by reflection from polished 

 surfaces. 



(3) The photographic effects of the electric spectra of different metals 

 talien in air, including 



a. Pare metals. b. Alloys. 



(4) Photographic effects of electric spectra of different metals produced by 

 transmitting the sparhs through gases other than atmospheric air. 



The general results having reference to (1) may be stated as 

 follows : — 



Colourless bodies which possess equal powers of transmitting the 

 luminous rays vary greatly in permeability to the invisible rays. 



Diactinic solids (that is to say, solids which are permeable to the 

 chemical rays) preserve their diactinic power both when liquefied and 

 when converted into vapour. 



Colourless solids which are transparent to light, but exei-t a con- 

 siderable absorptive effect upon the invisible rays, preserve their absorptive 

 power with greater or less intensity both in the liquid and the gaseous 

 state. 



In the pi'eparation of the various compounds for examination, much 

 care is stated to have been taken to employ materials in a state of purity. 

 Notwithstanding, in some cases there was reason to believe that some 

 impurity was present which could not be detected by the ordinary tests, 

 but which was opaque to the actinic rays. Subsequent researches by 

 others have pi'oved this surmise to be correct. It was found that filtration 

 through paper sensibly impaired the diactinicity of a solution. 



Dr. Miller states that he was unable to trace any special connec- 

 tion between the chemical complexity of a substance and its diactinic 

 power. 



As regards ' the absorption of the invisible rays by transmission through 

 different media,' it may be remarked that the solids examined were not 

 of uniform thickness, and the substances experimented on in a state of 

 solution were in the condition of saturation, and, therefore, not fairly 



' Annates de CJnmie, Ser. 3, vol. ix. p. .SOI. 



2 Phil. Trans. 1863. ^ £oc. cit. 1863. 



