TRANSACTIONS OF SECTION O, 723 



Deville lias investigated similar phenomena in chemical reactions ; he has found that 

 at certain temperatures and pressures substances fall asunder and combine much in 

 the way in which evaporation takes place, and has given the name of 'dissociation' 

 to this property of matter. Professor I\Iendel6eff and others have extended the 

 great Frencli chemist's observations, and have formulated the jreneral law that 

 substances are capable of dissociation at all temperatures, not only in the case of 

 chemical unions, but also in that of solutions. 



If steel 1)6 looked upon as a solution of carbon and iron, then the 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 pro- 

 perties of which is extreme 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 wlien suddenly cooled at any 

 point there remains again a distinct substance, composed of iron and carbon, partly 

 m various degrees of solution and partly free, and again possessing .'Jjiecial mechani- 

 cal qualities. That steel, and probably other alloys, differ in the nature of their 

 composition according to the way in which they are worked, both with respect to 

 heat and mechanical pressure, has been abundantly proved by many eminent metal- 

 lurgists, and especially by Sir Frederick Abel, in the extended researches which he 

 has recently carried out, on the hardening of steel, for the Institution of Mecha- 

 nical Engineers, and it would appear as a natural sequence that the properties of 

 steel would be greatly aflected by the manner in which it.i temperature was changed, 

 as we indeed find that it is when these changes are produced by baths of melted 

 metals, by oil, or by water at different temperatures. The action which takes 

 place may be illustrated by what would happen supposing that a complicated dance, 

 such as the Lancers, were suddenly stopped in various pha.«es of the figures. The 

 component parts would always remain the same, but the relative distribution of the 

 partners would vary continually, and analysis would .show that at one time each 

 gentleman was associated with a particular lady ; at another, that two ladie.< were 

 attached to a single gentleman, while a number of gentlemen had no partners at 

 all ; and yet, again, at another, that the movements which were once rectilinear 

 have become circular. In each case the groups would assume a totally distinct 

 appearance. 



In support of these views it may be stated that, as far as I know, no pure 

 element is capable of being hardened or tempered, the reason being that no 

 chemical change can take place when there is only one substance ; the effect of heat 

 or pressure, however suddenly applied, produces merely a change of form which 

 does not appear to carry with it any corresponding alteration of mechanical 

 properties. 



It may be urged, however, that it is unlikely that alloys or solutions could be 

 affected in a manner so marked merely by small changes at comparatively low 

 temperatures ; but 1 would observe that ' great and little ' are relative terms, 

 and we have abundant evidence of the immense effects produced by what 

 would be called ' little ' cau.ses. Sir Frederick Bramwell, in bis address last year, 

 drew attention to the importance of the ' next to nothing.' It is not so very long 

 ago that anyone would have been considered a dreamer for propounding a theory 

 that the presence of the fraction of a per cent, of carbon, phosphorus, or sulphur 

 would totally alter the character of iron ; that the addition of one two-thousandth 

 part of aluminium to molten iron would make the pasty mass as fluid as water; 

 that the presence of the smallest impurity in copper would have a disastrous effect 

 on its powers of conducting electricity ; and that the addition of one thousandth 

 part of antimony would convert the' ' best selected ' copper into the worst con- 

 ceivable. I need hardly allude to the great part played in nature by microscopic 

 organisms, and how much of the beauty of our seas "and rivers is derived from 

 substances so minute that nothing but the electric beam of Professor Tyndall is 

 capable of revealing their presence. 



There is one more circumstance connected with my subject to which I must 

 draw your attention, because, though its application to the mechanical properties 



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