Februaey 14, 1896.] 



SCIENCE. 



227 



In a later number of the Zeitschrift fur 

 physikalische Cheinie, Ostwald reviews the sec- 

 ond edition of J. B. Stallo's ' The Concepts 

 and Theories of Modern Physics ' that ap- 

 peared in 1885, and expresses the hope that 

 the book may find half as many readers as 

 it deserves. The book was first issued in 

 1882 as one of the International Scientific 

 Series, and scientific men as a whole re- 

 garded it unfavorably, though some of them 

 certainly recognized the force of many of 

 the author's arguments against the mate- 

 rialistic conceptions which were then and 

 are now generally held. 



Iea Kemsen. 



ON A NEW KIND OF BAYS* 



1. A DISCHARGE from a large induction 

 coil is passed through a Hittorf 's vacuum 

 tube, or through a well-exhausted Crookes' 

 or Lenard's tube. The tube is surrounded 

 by a fairly close-fitting shield of black paper; 

 it is then possible to see, in a completely 

 darkened room, that paper covered on one 

 side with barium platinocyanide lights up 

 with brilliant fluorescence when brought 

 into the neighborhood of the tube, whether 

 the painted side or the other be turned 

 towards the tube. The fluorescence is still 

 visible at two metres distance. It is easy 

 to show that the origin of the fluorescence 

 lies within the vacuum, tube. 



2. It is seen, therefore, that some agent 

 is capable of penetrating black cardboard 

 which is quite opaque to ultra-violet light, 

 sunUght or arc-light. It is therefore of in- 

 terest to investigate how far other bodies 

 can be penetrated by the same agent. It is 

 readily shown that all bodies possess this 

 same transparency, but in very varying de- 

 grees. For example, paper is very trans- 

 parent ; the fluorescent screen will light up 

 when placed behind a book of a thousand 



* From the translation in Nature by Arthur Stanton 

 from the Sitzungslerichte der Wurzburger Fhysik-medic. 

 Gesellsehaft, 1895. 



pages; printer's ink ofTers no marked re- 

 sistence. Similarly the fluorescence shows 

 behind two packs of cards ; a single card 

 does not visibly diminish the brilliancy of 

 the light. So, again, a single thickness of 

 tinfoil hardly casts a shadow on the screen ; 

 several have to be superposed to produce a 

 marked efiect. Thick blocks of wood are 

 still transparent. Boards of pine two or 

 three centimetres thick absorb only very 

 little. A piece of sheet aluminium, 15 mm. 

 thick, still allowed the X-rays (as I will 

 call the rays, for the sake of brevity) to pass, 

 but greatly reduced the fluorescence. Glass 

 plates of similar thickness behave similarly; 

 lead glass is, however, much more opaque 

 than glass free from lead. Ebonite several 

 centimetres thick is transparent. If the 

 hand be held before the fluorescent screen, 

 the shadow shows the bones darkly, with 

 only faint outlines of the surrounding 

 tissues. 



Water and several other fluids are 

 very transparent. Hydrogen is not mark- 

 edly more permeable than air. Plates of 

 copper, silver, lead, gold and platinum also 

 allow the rays to pass, but only when the 

 metal is thin. Platinum .2 mm. thick al- 

 lows some rays to pass ; silver and copper 

 are more transparent. Lead 1.5 mm. thick 

 is practically opaque. If a square rod of 

 wood 20 mm. in the side be painted on one 

 face with white lead it casts little shadow 

 when it is so turned that the painted face 

 is parallell to the X-rays, but a strong 

 shadow if the rays have to pass through the 

 painted side. The salts of the metal, either 

 solid or in solution, behave generally as the 

 metals themselves. 



3. The preceding experiments lead to 

 the conclusion that the density of the bodies 

 is the property whose variation mainly 

 affects their permeability. At least no 

 other property seems so marked in this con- 

 nection. But that the density alone does 

 not determine the transparency is shown by 



