ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 175 



micro-organisms actually diminished. After being kept fifteen days 

 the water thus treated was found to contain only two micro-organisms 

 to 1 c.cm. Hence the results of these experiments leave no doubt 

 that carbonic acid is an impediment to the existence of micro-organisms 

 in potable water. The practical importance of this of course is 

 obvious, and needs no comment for those who are accustomed to 

 drink waters " aerated " with carbonic acid, for according to Dr. 

 Leone, the longer these aerated waters are kept the less chance there 

 is of their being contaminated with bacterial impurities. 



Microscopical Structure of Iron and Steel.* — Dr. H. C. Sorby 

 has dealt with this subject in a paper read before the Iron and 

 Steel Institute, and from which we extract the parts which refer to 

 the preparation of the objects and their illumination. 



The microscopical study of fractured surfaces is, he considers, 

 unsatisfactory, not only on account of the optical difficulties, but 

 because a fracture shows the line of weakness between the crystals, 

 and not their internal structure. All his results were therefore based 

 on the examination of flat sections. These should be finished by 

 grinding with Water of Ayr stone, and polished so as not to alter the 

 true structure of the extreme surface. Anything approaching to a 

 burnished surface or polished scratches is fatal to good results. In 

 general, after having been polished with the finest rouge and water, 

 so as to show few or no scratches, the surface was acted on by very 

 dilute nitric acid, and repeatedly examined in a small trough of water, 

 until it was found that the acid had properly developed the structure. 

 In some cases it is, however, best to polish with dry rouge on parch- 

 ment, and not to use acid. Thin glass covers were afterwards mounted 

 over the surface with Canada balsam. Some of his preparations 

 have kept perfectly well for above twenty years, but others have 

 deteriorated considerably. 



Objects thus prepared must be examined by means of two special 

 kinds of surface illumination, viz. first, the side parabolic reflector 

 now common, but the author believes originally made for this purpose, 

 which gives oblique light, and secondly, a small silver reflector, 

 covering half the object-glass, which throws the light directly down 

 on the object, and from this it is reflected back through the other half 

 of the lens (see supra p. 130, fig. 14). With the oblique illumination, 

 a polished surface looks black, but with the direct illumination it 

 looks bright and metallic. A truly black substance appears black in 

 both cases. A magnifying power of about sixty linear is most 

 generally suitable, but the sections will bear a higher perfectly well. 



In commenting on a paper on the properties of malleable iron by 

 Dr. H. Wedding, Dr. Sorby wrote f : — " As far as I can judge the 

 reason why his (Dr. Wedding's) conclusions differ so much from mine 

 is that his sections were not ground down with soft stone before 

 final polishing. It was not till I adopted this method that I was 

 able to see the ultimate structure properly. This explains why he 



* Sorby, H. C, 'On the Microscopical Structure of Iron and Steel,' 8vo, 

 Iron and Steel Institute, 1885, 8 pp. 



t Colliery Guardian, xlix. (1885) p. 908. 



