234 SUMMA.RY OF CURRENT RESEARCHES RELATING TO 



tiou, it had been found preferable to employ an extemporized device, 

 consisting of a large-sized microscope-slide on which columns ^ mm. 

 in width had been ruled. On this a ring of paraffin-wax, applied while 

 molten by means of a turn-table, was placed. A definite volume — say, 

 10, 20, or more c.mm.^of the suspension of organisms to be counted 

 was expelled from a graduated capillary pipette on to the ruled slide, 

 and a cover-slip was allowed to fall upon it. The organisms in the whole 

 of the drop were easily and rapidly counted by working along the columns 

 from end to end. The chamber and ruled slides could be obtained from 

 Messrs. H. F. Angus and Co., Wigmore Street, W. 



Metallography, etc. 



Comparison Screen for Brass.* — A simple method for determining 

 approximately the grain-size of annealed brass is described and illus- 

 trated by 0. W. Ellis. A glass screen is employed carrying prints on 

 transfer-paper of photomicrographs of standard samples which have 

 been subjected to different but known heat-treatments, arranged in the 

 form of a ring. The image of the structure of the sample under 

 examination is projected on to the centre of the screen, and direct 

 comparison is then readily made with the standard samples. 



Effect of Great Hydrostatic Pressure on Metals. j — Z. Jeffries has 

 found that cylinders of pure aluminium and of an aluminium-copper 

 alloy (88 p.c. aluminium), after subjecting to hydrostatic pressure up 

 to 12,400 kilograms per square centimetre at temperatures of 25° and 

 40°C.,are practically unchanged in dimensions, or in regard to hardness, 

 strength, and microstructure. Both the metals tested had fine-grained 

 structures, and owing to the haphazard orientation of the crystals it is 

 considered that the resistance to deformation would be equal in all 

 directions, and hence no permanent deformation would result from 

 great hydrostatic jjressure. A single crystal or a piece of metal com- 

 posed of a few larger grains, on the other hand, would show diiferent 

 degrees of resistaoce to deformation in different directions, and might 

 suffer permanent deformation under great hydrostatic pressures. 



Etching with Chromic Acid and Hydrogen Peroxide.| — This 

 mixture is stated by S. AV. Miller to be a useful etching reagent for all 

 bronzes and brasses, and also for silver. The reagent is made by adding 

 a few drops of hydrogen peroxide to a very dilute solution of chromic 

 acid. Addition of hydrogen peroxide causes strong effervescence, and 

 turns the solution a very dark brown colour. If the chromic acid solu- 

 tion is too strong large gas-bubbles are evolved which prevent uniform 

 etching. The specimen is immersed in the effervescent solution for a 

 few seconds and then washed immediately in running water. Photo- 



* Journ. Inst. Metals, xviii. (1917, 2) pp. 171-2 (1 fig.). 



t Journ. Inst. Metals, xviii. (1917,2) pp. 243-7. 



J Journ. Inst. Metals, xviii. (1917, 2) pp. 253-5 (2 figs.). 



