1892.] on Metah at High Temperatures, 511 



attain a state of equilibriura, and this conflict is but a type of the 

 action that takes place in many metals and alloys which are of vast 

 industrial importance. 



Time will only permit me to deal with three cases of the action of 

 high temperatures on atoms and molecules of metals. In the first 

 case, the arrangement of the atoms in the molecule of a metal, iron, is 

 disturbed, and the result is of great industrial importance. In the 

 second case, the atoms of a metal, gold, appear to combine with those 

 of another metal ; and the result, while it is mainly of interest in 

 connection with the history of science, has nevertheless an important 

 bearing upon art. The third case relates to the molecular bombard- 

 ment which takes place when a small quantity of metal is dissolved 

 in a mass of metallic solvent, and is of interest in connection with 

 modern views both as to osmotic pressure and solution generally. 



(1) The pyrometric couple is inserted in the centre of a little 

 mass of steel, which is being slowly raised to a bright red heat ; 

 when the flame is withdrawn, the spot of light will turn towards the 

 zero end of the scale, falling slowly until a temperature of 655^ is 

 reached, and then there will be an abrupt and prolonged arrest. The 

 metal has never been near its melting-point, and the evollition of heat 

 must be due to a molecular change in the solid metal. In the case of 

 this particular sample of steel, the evolution of heat is mainly the 

 result of a change in the relation between the carbon and the iron; 

 but by laboratory experiments and careful chronographic records, 

 Osmond has shown that, in the case of certain varieties of steel, it 

 can be demonstrated that what here appears as a single change, 

 attended by an evolution of heat, is really an exceedingly complex 

 one. I have shown that it occurs in the purest iron the chemists can 

 prepare by electrolysis, and I agree with Osmond in believing that 

 the change which occurs in pure iron at 855^ is a molecular one, 

 independent of the presence of impurity. If the mass of steel 

 (Fig. 5, a) be heated again and allowed to cool, you will observe that 

 the point of " recalescence " appears to be that at which the iron 

 regains its magnetic property ; * for a magnetised needle, h, is 

 attracted at the moment the arrest of the spot of light on the pyro- 

 meter scale marks the temperature at which the change occurs, and 

 at that precise moment a second spot of light, from a mirror mounted 

 on the magnetic needle, will rapidly move away from its zero. I 

 have elsewheref dwelt on the importance of the molecular change in 

 iron and steel, and can now only summarise the significant facts. 



It is unnecessary to point to the extreme industrial importance 



* The temperature at which these molecular changes take place in iron aud 

 steel was first demonstrated to an audience in my Newcastle lecture, 1889; but 

 my friend Prcjf. Eeinold, of the Royal Naval College, first arranged an experi- 

 ment for lecture purposes, which showed the magnetic change simultaneously 

 wih the thermal one. 



t Report to the Institution of Mechanical Engineers, 'Proceedings,' 1891, 

 p. 543. 



