138 THE X-RAYS. 



distinctly on the screen. Thick wooden blocks are transmissible. Slabs 

 of pine 2 or 3 centimeter.s thick absorb only very little. A plate of 

 aluminum about 15 millimeters thick diminished the effect very con- 

 siderably, but did not cause the fluorescence to entirely disappear. 

 Blocks of hard rubber several centimeters thick still transmitted the 

 rays.'^ 



Glass plates of equal thickness behave very differently according to 

 whether they contain lead (flint glass) or not. The first class are much 

 less transmissible than the second. 



If the hand is held between the vacuum tube and the screen, the 

 dark shadow of the bones is seen upon the much lighter shadow out- 

 line of the hand. Water, carbon bisulphide, and various other liquids 

 investigated proved very transmissible. I could not find that hydrogen 

 was more transmissible than air. The fluoresence was visible behind 

 plates of copper, silver, lead, gold, and platinum, when the thickness 

 of the plate was not too great. Platinum 0.2 millimeter thick is still 

 transmissible, and silver and copper plates may be still thicker. Lead 

 1.5 millimeters thick is practically imj)enetrable, and advantage was 

 frequently taken of this characteristic. A wooden stick of 20 milli- 

 meters square cross section, having one side covered with wliite lead, 

 bebaved difl'erently when interposed between the vacuum tube and the 

 screen according as the X-rays traversed the block parallel to the 

 painted side or were compelled to pass through it. In the first case 

 there was no oifect appreciable, while in the second a dark shadow was 

 thrown on the screen. Salts of the metals, whether solid or in solution, 

 are to be ranged in almost the same order as the metals themselves for 

 transmissibility. 



3. These observations and others lead to the conclusion that the 

 transmissibility of equal thicknesses of different substances depends 

 on their density. At least no other characteristic exerts so marked an 

 influence as this. 



The following experiment shows, however, that the density is not the 

 sole factor. I compared the transmissibility of nearly equally thick 

 plates of glass, aluminum, calcspar, and quartz. The density of these 

 substances is substantially the same, and yet it was quite evident that 

 the calcsTiav was considerably less transmissible than the others, which 

 are about alike in this respect. 



4. All bodies became less transmissible with increasing thickness. 

 For the purpose of finding a relation betweeu transmissibility and 

 thickness I have made photographic exposures, in which the photo- 

 graphic i^late was partly covered with a layer of tin foil consisting of a 

 progressively increasing number of sheets. I shall make a photometric 

 measurement when I am in possession of a suitable photometer. 



5. Sheets were rolled from platinum, lead, zinc, and aluminum of such 



■^ For brevity's. sake I employ the word "rays," and in distiuction from others make 

 use of the exjiression "X-rays." 



