ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 605 



familiar, and Dr, E. Geinitz gives the results of an extended study of 

 the plagioclase rocks and phonolites of the Mecklenburg drift. 



The method consists in examining thin sections of the rocks 

 found in the drift, and comparing them with the descriptions given 

 by the Scandinavian lithologists of rocks known in situ in that penin- 

 sula. In this way various basalts, diabases, gabbros, diorites, and 

 phonolites are referred to certain localities in Sweden, whence they 

 are supposed to have been derived. Interesting results can be 

 obtained by such methods ; but they are often uncertain, since it 

 cannot be predicated that rocks of the same character do not exist, or 

 have not existed, in the intermediate drift or water-covered areas. 



Microscopical Analysis of the Structure of Iron and Steel.* — 

 The first step, writes Mr. J. C Bayles, to be taken in practical micro- 

 scopy is the training of the eye to observe what may be seen without 

 the aid of a lens. This is accomplished by the patient examination 

 of characteristic fractures, and noting similarities and differences. 

 After the naked eye has become familiarized with all it can see, the 

 student should continue his investigations assisted by a hand lens 

 with a power of from two to three diameters, and absolutely achromatic. 

 Specimens to be studied with a view to determining their internal 

 structures should be surfaced in a planer, and smoothed by draw- 

 filing in the direction of the fibre. The surface thus obtained is 

 treated with slightly diluted nitric acid, which gives a rapid and wide 

 development of the structure, which may be studied with advantage 

 while it lasts, and will prepare the student for finer work. For fine 

 development more care and time are needed. After planing, the 

 surface of the metal is ground with fine emery or under a metallic 

 mirror-grinder. It is then treated with acid. A thorough develop- 

 ment with weak acid requires from twenty-four hours to six days, 

 according to the composition of the metal. Small specimens are pre- 

 pared by planing down from the back to a thickness of l-32nd to 

 1-16 in. The planed face is then ground and surfaced on a fine 

 whetstone, developed with weak acid, and mounted between glasses with 

 Canada balsam. In selecting a Microscope, care should be taken that 

 the lenses give a good definition, that there is no " shake " or lateral 

 motion in the adjustments for focus, and then the table should admit 

 of inclination at any angle found most convenient for observation. 



Concerning the results to be expected from the microscopical 

 analysis of metals, Mr. Bayles expresses the belief that it opens a 

 vast field of knowledge not yet reached by either chemical analysis 

 or physical test. There are many conditions, the result of changes 

 produced by mechanical treatment, to which chemical analysis gives 

 no clue and which are detected, but not explained, by the test of 

 the physical laboratory. The Microscope will, no doubt, explain 

 many of the mysterious changes which occur in metals of given 

 chemical composition under different conditions, and will give the 

 metallurgist an opportunity of studying the anatomy and physiology 

 of iron and steel, which, in a most important sense, will supplement 



* Science, i. (1883) p. 101. 



