70 THE AMEEICAN MONTHLY [April, 



I find the method of embedding by the use of a coil of paper about the 

 cork troublesome, on account of the formation of air-bubbles. The simplest 

 and most effectual manner to fasten the specimens to corks is as follows : — 

 Soak the cork for a short time in absolute alcohol, then flow over the surface 

 on which you embed a film of thin celloidin, letting it partially harden. 

 Place the object in the position desired for sectioning by the aid of the amount 

 of celloidin that will adhere to it from the vial. Let this stiffen slightly, then 

 add, at intervals of a minute, a few drops of the celloidin (depending, of 

 course, upon the size of the object) by allowing it to flow over the object and 

 about the base of it. Repeat this until a fair amount is covering and sup- 

 porting the specimen. By this means you have only the required amount of 

 celloidin about the object to support it firmly, and not a large mass to draw 

 the knife through. After a few moments a film suflicientlv firm will have 

 formed to hold the object in position. The entire mass is now to be placed 

 in a jar of alcohol of So^V to harden — recjuiring 34 to 48 hours. If the cork 

 is shallow and broad no weights will be necessary ; merelv invert the object 

 in the alcohol and the cork will serve to float it and keep it immersed. 



The mass is now ready for the microtome, and the blade should be flooded 

 with commercial alcohol. After sections have been obtained the embedded 

 object can be returned to the So/o alcohol, when it can be preserved for future 

 use. 



In clearing sections avoid the employment of absolute alcohol (unless used 

 cautiously) or clove oil, as these agents rapidly dissolve the celloidin. Sec- 

 tions so embedded are best cleared in creosote and mounted in xylol balsam. 



24th January, 1888. 



On the Use of the Microscope iu Petrography. 



By WM. H. HOBBS, 



FELLOW IN GEOLOGY OP THE JOHNS HOPKINS UNIVERSITY. 



The brilliant results obtained at the opening of the present century by Wil- 

 liam Smith, the fiither of English geology, directed the attention of geologists 

 to an arrangement of the secondary formations in definite horizons, by a study 

 and comparison of their organic remains. The lively interest that was at once 

 aroused, and the promiseof important results, produced a mostdesirable change 

 in geological research. The bitterness and lack of candor, which had been 

 so characteristic of the controversy between the rival schools of Freyberg and 

 Edinburgh on the origin of trap rocks, became matters of less common occur- 

 rence and were gradually lost sight of in the newly-opened field of geological 

 inquiry. From the publication of Smith's geological map of England, in 1815, 

 to near the middle of the century, the study of the crystalline rocks, though 

 undertaken by some, received an amount of attention in no degree commen- 

 surate with that devoted to the fossiliferous strata. The great controversy of 

 the Neptunists and Vulcanists did not end, however, without yielding some 

 important results. Demarest had shown conclusively that the older rocks 

 of the Auvergne were the product of volcanic action, and Hutton clearly 

 proved in 17S5 the igneous origin of the granite of the Grampian Hills. In 

 order to understand the small results that were obtained at this period in the 

 study of the massive rocks, we have only to consider the difficulties which had 

 to be encountered. The coarse-grained rocks allowed a separation of their 

 component minerals which could then be tested as to hardness, specific grav- 

 ity, color, lustre, cleavage, streak, and, best of all, chemical composition. The 

 new formulation of the law of the constants of the interfacial angles of crystals 

 by Rome de ITsle and the invention of the goniometer afforded the most ac- 



