1896.] on Electric Shadows and Luminescence. 193 



cient for an exposure. Indeed one minute is too much for many 

 objects. I have not previously tried this particular tube, though I 

 judge by its appearance that it is in good condition. As soon as 

 the exposure of one minute is over we will have the plate taken into 

 the dark room and developed in the ordinary way ; and when it is 

 developed we will have it brought back into this room and put into 

 the lantern, that you may see what has been done. 



Now, while we are taking photographs, I may as well take a 

 second to illustrate another point. Roentgen investigated in the 

 most careful and elaborate way the relative transparency of different 

 materials for these mysterious rays. He noticed that wood, and 

 many substances which are opaque to ordinary light, are transparent 

 to these rays ; whilst, on the contrary, several substances that are 

 transparent to light, such as calc-spar and heavy glass, are very 

 opaque toward them. Many experimenters have examined this 

 question of relative transparency. I devoted a day or two to the 

 study of gems, and found that imitation rubies made of red glass are 

 much more opaque than real rubies, and that paste diamonds are 

 much more opaque than real diamonds. Real diamonds and rubies 

 are indeed very transparent, and scarcely cast any shadows on the 

 luminescent screen, though I have found diamond to be more opaque 

 than an equal thickness of black carbon. There are laid upon this 

 piece of card two rubies, one being only a glass ruby. There is~al«o 

 a row of four small diamonds. I will leave you to find out whether 

 they are false or real. And then there are three larger diamonds, 

 one of which is uncut and is a genuine South African stone. I lay 

 them down upon a photographic plate and expose them to the 

 Roentgen rays so that we may test their relative transparency. 

 (The two photographs thus taken were projected upon the screen at 

 the close of the lecture.) 



Amongst the things which Roentgen told us was the fact that 

 different kinds of glass are unequally transparent : that lead-glass, 

 for instance, is much more opaque than soda-glass, or potash-glass, or, 

 indeed, any glass which does not contain a heavy metal like lead. 

 He found that practically the transparency was governed by the 

 density; that the heavy or the dense substances were the more 

 opaque. There is now some reason to correct that statement, though 

 in the main as a first approximation it is perfectly true. Professor 

 Dewar has shown that you must take into account, not the density in 

 gross but the atomic weight. Taking any homologous series, for 

 example, such as a number of sulphides, or oxides, or chlorides, that 

 one which contains the atomically heavier metal will be the more 

 opaque. Again, the bromide of sodium is more opaque than the 

 chloride of the same metal, and the iodide is more opaque than the 

 bromide. But as the correspondence between relative opacity and 

 molecular or atomic weight breaks down when we try to pass from 

 one series of compounds to a different series, there is some reason to 

 carry the matter to a further degree of approximation. We must go 



Vol. XV. (No. 90.) o 



