ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 515 



librium curves. The end of the eutectic line lies at 2 • 07 p.c. carbon ; 

 the eutectic composition is 4*3 p.c. carbon. A calculation of the 

 molecular weight of carbon dissolved in iron, based on Rothmund's 

 equation 



A t = E gl ~ Cz 



m 



is given, but the results are doubtful owing to the uncertainty which 

 exists as to the value of the latent heat of fusion of iron. Assuming it 

 to be 20, the molecule of carbon dissolved in iron appears to contain 

 2 atoms. 



An Etching Method.* — J. A. Aupperle points out that determina- 

 tions of silicon and oxygen in steel, supposed to distinguish crucible steel 

 from that made by other processes, cannot be relied on to do this. He 

 has employed the following etching method : Specimens of the steel 

 1| in. square are placed in dilute sulphuric acid containing perman- 

 ganate of potash (90 c.cm. water, 10 c.cm. 1*84 s.g. acid, 3 grm. 

 permanganate, for each specimen). The action is allowed to proceed 

 over night ; if the solution becomes coloured it is boiled until clear, 

 more acid being added if necessary. The pieces are washed in water, 

 and carbonaceous matter is wiped off. By this treatment open hearth 

 and Bessemer steel are deeply etched in grooves : the edge is honey- 

 combed and rough to the touch. Crucible steel is not etched in grooves 

 but shows a close structure : the edges are smooth. 



Solidification of Copper. f — P. Dejean has studied the freezing point 

 of copper with the object of deciding as to its suitability as a fixed point 

 for the calibration of thermo-couples. Platinum, platinum-iridium 

 couples were used, standardised daily by determining the melting point 

 of a small quantity of gold wire, this being taken as 1065°. Other fixed 

 points taken were the boiling point of sulphur 445°, and the solidifying 

 point of aluminium 655°. The cooling curves were photographically 

 recorded, three galvanometers being used on the induction system 

 previously described by the author 4 About 100 grm. pure copper, 

 deoxidised by hydrogen, was melted under wood charcoal, and a cooling 

 curve taken. Similar determinations of freezing points were made on 

 10 samples containing varying quantities of oxide, which was sub- 

 sequently estimated. Each of these samples gave two freezing points 

 until the eutectic composition was reached. As a check, the solidifying 

 points of several copper-aluminium alloys were determined. The results 

 given by the author are : — 



Freezing point pure copper, 1085°. 



Freezing point eutectic (copper, copper oxide), 1065°. 



Composition of eutectic, 4 ■ 5-5 ' p.c. oxide. 

 The freezing point of the eutectic is somewhat lower when it is present 

 only in small quantities. Its composition was verified by microscopic 

 examination. The lower freezing point of copper containing 2-3 p.c. 



* Iron and Steel Mag., xi. (1906) pp. 383-5 (2 figs.), 

 t Rev. Metallurgie, iii. (1906) pp. 233-42 (10 figs.). 

 J See this Journal, 1905, p. 777. 



