R. M. Field — Roentgen Ray in Paleontology. 545 



Another interesting table is to be found in vol. 50 of the 

 International Library of Technology, page 71. In this table 

 the transparencies (penetrabilities) of the various minerals and 

 substances are compared with that of water. Only the figures 

 for water, glass and the metals are given below : 



Substance Specific gravity Transparency 



Water l'OO 1*000 



Aluminium 2-67 '380 



Glass 270 -340 



Tin 7-29 -118 



Zinc 7*16 -116 



Iron 7'78 -101 



Nickel -. 8-51 -095 



Copper 892 -084 



Silver 1 0'24 -070 



Lead 11-39 -055 



Mercury... 13-59 -044 



Gold.. 19-63 -030 



Platinum 21*53 -020 



From these tables we learn at a glance that a large number 

 of inorganic substances may be distinguished from each other 

 by means of the Roentgen apparatus, and the intensity of 

 their shadows registered by skiagraphy. 



Doelter's table is interesting in that it shows the carbonace- 

 ous substances to be the ones which are the most easily pene- 

 trated by the ray. On the other hand, the metallic sulphides 

 are the most impenetrable. Again, gypsum, chalk and ara- 

 gonite are penetrated with more difficulty than kaolin, quartz 

 and plagioclase. Obviously, the different effects of the ray on 

 the sulphides of the metals, marcasite and pyrite, from those 

 upon calcite, aragonite and quartz, are of especial interest to 

 us in connection with the investigation of structures in fossils. 

 If we now turn our attention to the mineral composition of 

 fossils and fossiliferous rocks, we see at once the possibility of 

 determining, by means of the ray, not only the presence of fos- 

 sils hidden in the rocks, but also, which is more important, we 

 should be able to discover the internal structures of the fossils 

 themselves, when such structures have been preserved. As has 

 already been stated, Branco,* inspired by Doelter's earlier 

 work, made numerous experiments with both vertebrate and 

 invertebrate fossils. His well-illustrated paper proves without 

 doubt the possible application of skiagraphy to fossil diagnosis. 

 Many of Doelter's experiments have been independently 

 proved by the writer. Moreover, the writer's experiments 

 with brachiopods were especially productive of interesting 

 results and it is in the study of this class of invertebrate fossils, 



* " Die Anwendung der Roentgenstrahlen in der Palaeontologie." Abhand. 

 Berlin. Akad., 1906. 



