Geology and Mineralogy. 229 



current is reversed a hundred million times a second as for steady 

 currents." — Nature, Jan. 24, 1889. j. t. 



17. Orthochromatic Photography. — H. W. Vogel criticises 

 Abney's method of sensitizing a plate, which consists in flowing 

 the dry plate with a colored collodion, and claims that it does not 

 produce the same effect as the direct mixture of the coloring mat- 

 ter with the gelatine layer of the dry plate. The collodion film 

 prevents the molecular action of the sensitizing substance upon 

 the bromide of silver molecules. — Photog. Mittheil, xxv, p. 117- 

 119, 1888. j. t. 



18. Voltaic Balance. — Mr. G. Gore finds the following appa- 

 ratus extremely sensitive. An unamalgamated zinc plate and a 

 platinum plate are immersed in a small vessel of distilled water 

 and are connected, through a galvanometer, with a platinum and 

 a zinc plate in another vessel filled with distilled water. The 

 smallest addition of a foreign substance to one or the other vessel 

 causes swing of the galvanometer, and the apparatus can be used 

 to detect extremely small traces of various substances. — Chem. 

 News, lviii, p. 64, 1888. j. t. 



II. Geology and Mineralogy. 



I. Fossil Plants of the Coal-measures of Rhode Island ; by Leo 

 Lesqttereux. (Communicated by the author). — The following is a 

 list of the coal plants of Rhode Island, sent for determination by 

 the Museum of Brown University, Providence, R. I., May, 1888. 



1. Pecopteris dentata Brgt., 1 a fruiting pinna, Pawtucket. 



2. Sphenpoteris (Hymenophyllites) furcata Brgt., Valley Falls. 



3. Sphenophyllum oblongifolium Germ., Pawtucket. 



4. Dictyopteris Scheuchzeri Hoffm. ? obscure ; nervation obsolete, Pawtucket. 



5. Odontopteris Stiehlerian Goepp., fragment, Pawtucket. 



6. Odontopteris Reichiana Goepp., separate pinnule, Pawtucket. 



7. Odontopteris Reichiana, var. latifolia Lx., Pawtucket. 



8. Odontopteris Neuropteroides Roem , Pawtucket. 



9. Neuropteris decipiens Lx., Valley Palls. 



10. Goniopteris (Pecopteris) unita Brgt., Valley Falls. 



II. Pecopteris lepidorachis Brgt., Valley Falls. 



12. Asterophyllites equisetiformis Brgt., Pawtucket. 



13. Pecopteris Miltoni Brgt, Pawtucket. 



14. Schizopteris (Rhacophyllum) trichomanoides Goepp., Pawtucket. 



15. Oligocarpia Gutbieri Goepp., Pawtucket. 



16. Sphenopteris lanceolata Gutb., Pawtucket. 



17. Odontopteris Bairdii Brgt., Pawtucket. 



18. Pecopteris he miteloides Brgt.,? fruiting pinna, obscure, Pawtucket. 



19. Pecopteris Miltoni Brgt, Pawtucket. 



20. Pecopteris abbreviata Brgt., fruiting fragment, Pawtucket. 



21. Pecopteris arborescens Brgt., fruiting fragments, Pawtucket. 



22. Pseudopecopteris dimorpha Lx., fragment of a pinna, Bristol. 

 22a Pseudopecopteris dimorpha Lx., obsolete form, Bristol. 



23. Odontopteris cornuta Lx., Pawtucket. 



24. Parallel narrow rachises of pinnae of No. 7, mostly deprived of leaves, 

 Pawtucket. 



25. Neuropteris dentata Lx. See remarks below, Pawtucket. 



26. Odontopteris obtusiloba ? Baum. 



