716 THE MOLECULAR STRUCTURE OF MATTER. 



If steel be looked upon as a solution of carbon and iron, then tlie 

 hardening- of steel is explained by the theory that dissociation has taken 

 place at the temperature at which it is suddenly cooled, the sudden 

 cooling fixing the molecular motion at such an amplitude or phase that 

 it gives a characteristic structure, one of the properties of which is ex- 

 treme hardness. In tempering, the gradual communication of heat 

 causes dissociation again to take place, the molecular equilibrium is 

 modified by the increased energy imparted to the particles, and when 

 suddenly cooled at any point there remains again a distinct substance, 

 composed of iron and carbon, partly in various degrees of solution and 

 partly free, and again possessing special mechanical qualities. - - - 



There is one more circumstance connected with my subject to which 

 I mnst draw your attention, because, though its application to the me- 

 chanical properties of substances is very recent, it i)romises to be of 

 great importance. I alhide to the Periodic Law of Dr. Mendel^ef 

 According to that law, the elements arranged in order of their atomic 

 weights, exhibit an evident periodicity of properties, and as Professor 

 Carnelley has observed, the properties of the compounds of the ele- 

 ments are a periodic function of the atomic weights of their constitu- 

 ent elements. Acting on these views. Professor Roberts-Austin has 

 recently devoted much time and labor to testing their exactness with 

 reference to the mechanical properties of metals. The investigation is 

 surrounded by extraordinary difdculties, because one of the essential 

 features of the inquiry is that the metals operated on should be abso- 

 lutely pure. For chemical researches, a few grains of a substance are 

 all that is needed, and the requisite purity can be obtained at a mod- 

 erate cost of time and labor; but when mechanical properties have to 

 be determined, considerable masses are needed, and the funds necessary 

 for obtaining these are beyond the reach of most private individuals. 



In view of the dit^culty of obtaining metals of sufficient purity, he 

 selected gold as his base, because that metal can be more readily 

 brought to a state of purity than any other, and is not liable to oxida- 

 tion. In a communication to the Eoyal Society made last year, he 

 shows that the metals alloyed with gold which diminish its tenacity 

 and extensibility have high atomic volumes, while those which increase 

 these properties have either the same atomic volumes as gold or have 

 lower ones. The inquiry has only just been commenced, but it appears 

 to me to promise results which, to the engineer, will prove as important 

 and as fruitful of progress as the great generalization of Meudeleef 

 has been to chemists. A law which can not only indicate the existence 

 of unknown elements, but which can also define their proi)erties before 

 they are discovered, if capable of application to metalhirgy, must surely 

 yield most valuable results, and will make the compounding of alloys 

 a scientific process instead of the lawless and haphazard operation 

 which it is now. 



