August 17, 1905] 



NA TURE 



familiar. These are for the most part changes which are 

 special to particular elements or compounds, and are 

 usually classed with the chemical properties bv which the 

 substances may be distinguished from each other. Very 

 different is the amorphous crystalline change, for although 

 in particular cases it may have been observed and asso- 

 ciated with allotropic changes, yet the causes of its occur- 

 rence are more deeply founded in the relations between the 

 molecules and the heat energy by which their manifold 

 properties are successively unfolded as temperature is raised 

 from the absolute zero. At this transition point we find 

 ourselves face to face with the first stirrings of a specific 

 directive force by which the blind cohesion of the mole- 

 cules is ordered and directed to the building up of the 

 most perfect geometric forms. It is hardly possible any 

 longer to regard the stability of a crystal as static and 

 inert, and independent of temperature ; rather must its 

 structure and symmetry be taken as the outward manifesta- 

 tion of a dynamic equilibrium between the primitive co- 

 hesion and the kinetic energv imparted, by heat. Even 

 before the discovery of a definite temperature of transition 

 from the amorphous to the crystalline phase we had in our 

 hands the proofs that in certain cases the crystalline state 

 can be a state of dynamic, rather than of static equilibrium. 

 The transition of sulphur from the rhombic to the prismatic 

 form supplies an example of crystalline stability which 

 persists only between certain narrow limits of temperature. 

 Within these limits the crystal is a " living crystal " if 

 one may borrow an analogy from the organic world. It 

 can still grow, and it will under proper conditions repair 

 any damage it may receive. 



The passage; of the same substance through several crys- 

 talline phases, each only stable over a limited range of 

 temperature, strongly supports the general conclusion 

 drawn from the existence of a stability temperature between 

 the amorphous and crystalline phases, namelv, that the 

 crystalline arrangement of the molecules requires for its 

 active existence the particular kind or rate of vibration 

 corresponding with a certain range of temperature. Below 

 this point the crystal may become to all appearance a 

 mere pseudomorph with no pow.-rs of active growth or 

 repair. But these powers are not extinct — they are only in 

 abeyance ready to be called forth under the energising 

 influence of heat. This temporary abeyance of the more 

 active properties of matter is strikingly illustrated by the 

 early observations of Sir James Dewar at the boiling point 

 of liquid air, and more recently at that of liquid hydrogen. 

 At th? latter temperature even chemical affinitv becomes 

 latent. In metals it was found that the changes in their 

 physical properties brought about by these low temperatures 

 are not permanent, but only persist so long as the low 

 temperature is maintained. During the past year Mr. 

 R. A. Hadfield has supplemented these earlier results by 

 making a very complete series of observations on the effect 

 of cooling on the mechanical properties of iron and its 

 alloys. The tenacity and hardness of the pure metal and its 

 alloys at the ordinary temperature and at — 182° have been 

 compared, and it has been found that these qualities are 

 invariably enhanced at the lower temperature, but that 

 they return exactly to their former value at the ordinary 

 temperature. By the mere abstraction of heat between the 

 temperatures of 18° and —182° the tensile strength of pure 

 metals is raised 50 to 100 per cent. In pure iron the in- 

 crease is from 23 tons per square inch at 18° C. to 52 

 tons at —182°; in gold from 15-1 tons to 224 tons; and 

 in copper from 19-5 tons to 26-4. This increase is not, I 

 think, due to the closer approximation of the molecules, 

 for the coefficient of expansion of most metals below 0° 

 is extremely small. Neither is it due to permanent changes 

 of molecular arrangement or aggregation, for Mr. Hadfield 

 has obtained a perfectly smooth and regular cooling . urve 

 for iron between iS° and -182°, and there appears to be 

 no indication of the existence of any critical point between 

 these temperatures. Further, the complete restoration of the 

 original tenacity on the return to the higher temperature 

 shows that no permanent or irreversible change has occurred 

 during cooling. Everything therefore indicates that the 

 increase of tenacity which occurs degree by degree as beat 

 is removed is due to the reduction of the repulsive force 

 of molecular vibration, so that the primary cohesive force 



NO. 1868, VOL. 72] 



can assert itself more and more completely as the absolute 

 zero is approached. 



The metals experimented with by Mr. Hadfield were all 

 in the annealed or crystalline condition, so that the 

 molecules must have exerted .their mutual attractions along 

 the directed axes proper to this state. It is to be expected 

 that similar experiments with the metals in the amorphous 

 state may throw light; on the question whether and to 

 what extent the crystalline state depends on a dynamic 

 equilibrium between the forces of cohesion and repulsion, 

 or whether a directed cohesion exists fully developed in 

 the molecules at the absolute zero." 



The phenomena of the solid state throw an interesting 

 light on the interplay of the two great forces, the primative 

 or blind cohesion which holds undisputed sway at the 

 absolute zero, and the repulsion due to the molecular vi- 

 brations which is developed by heat. This interplav we 

 know continues through the states which succeed each 

 other as the temperature is raised, until a point is reached 

 at which the molecular repulsions so far outweigh the 

 cohesive force that the substance behaves like a perfect gas. 

 The problems of molecular constitution are more likely to 

 be elucidated by a study of the successive states between 

 the absolute zero and the vaporising temperature than at 

 the upper ranges where the gaseous state alone prevails. 

 The simplicity of the laws which govern the physical 

 behaviour of a perfect gas is very attractive, but we must 

 not forget that this simplicity is only possible because 

 repulsion has so nearly overcome cohesion that the latter 

 may be practically ignored. The attractiveness of this 

 simplicity should not blind us to the fact that it is in the 

 middle region, where the opposing forces are more nearly 

 equal, that the most interesting and illuminating phenomena 

 are likely to abound. The application of the gas laws to 

 the phenomena of solution and osmosis appears to be one 

 of those cases in which an attractive appearance of sim- 

 plicity in the apparent relations may prove very misleading. 



Before passing from the specially metallic qualities of 

 gold I will only remind you of the important part it has 

 played in the researches on the diffusion of metals by the 

 late .Sir William Roberts-Austen, and in those of Mr. 

 Haycock and Mr. Neville on the freezing points of solu- 

 tions of gold in tin, which led to the recognition of the 

 monatomic nature of the molecules of melals. 



Molecules in Solution. 

 It has occurred to me that the practice of the cyanide 

 process of gold extraction presents us with several new 

 and interesting aspects of the problems of solution. As 

 you are aware, the gold is first obtained from the ore in 

 the form of a very dilute solution of cyanide of gold and 

 potassium from which the metal has to be separated, either 

 by passing it through boxes filled with zinc shavings, or 

 by electrolysis in large cells. 



The solution as it leaves the cyanide-vats may contain 

 gold equal to 100 grains or more per ton, and as it leaves 

 the precipitating-boxes it may contain as little as i or 

 2 grains and as much as 20 grains. In the treatment of 

 slimes much larger volumes of solution have to be dealt 

 with, and in this case solutions containing 18 grains per 

 ton have been regularly passed through the precipitating- 

 boxes, their gold content being reduced to li grains per 

 ton. In round numbers we may say that i gram of gold 

 is recovered from i cubic metre of solution, while o-i gram 

 is left in the solution. Even from the point of view of the 

 physical chemist we are here in presence of solutions of a 

 very remarkable order of dilution. A solution containing 

 I gram per cubic metre is in round numbers N/20p,ooo, 

 and the weaker solution containing 01 gram is 

 N/2,000,000. It is convenient to remember that the latter 

 contains a little more than i^ grains per ton. In experi- 

 ments on the properties of dilute solutions the extreme 

 point of dilution was reached by Kohlrausch, who emploved 

 solutions containing 1/100,000 of a gram-molecule of solute 

 per litre for his conductivity experiments. These solutions 

 were therefore twice as strong as the gold solution with 

 I gram per cubic metre, and twenty times as strong as the 

 1 Since the above was wriuen a series of observations has been made on 

 the influence of low temperature on the tenacity of pure metals in the 

 amorphous condition. These observations will form the subject of a separate 

 communication to the Section. 



