828 



SCIENCE. 



[N. S. Vol. III. No. 75. 



and the corrections to be made are described 

 at length, and the values of the diflusivity 

 of various metals in lead are then given. 



The values for k, the dift'usivity, given in 

 sq. cm. per day, are as follows : 

 k 



Gold in lead 3-19 at 500°. 



" " bismuth 4'52 " " 



" " tin 4-65 " " 



Silver in tin 4']4 " " 



Lead in tin 3-18" " 



Rhodium in lead 3'04 " " 



Platinum in lead 1-69" 490°. 



Gold in lead 303 " " 



Gold in mercury 0"72 " 11°. 



In order to afford a term of compai'ison, 

 it may be stated that the diffusivity of 

 chloride of sodium in water at 18° is 1.04. 



The author at present refrains from draw- 

 ing any conclusion as to the evidence which 

 the results afford respecting the molecular 

 constitution of metals. It is, however, evi- 

 dent that they will be of value in this con- 

 nection, because, with the exception of the 

 gases, they present the simplest possible 

 case of diffusion which can occur — the diffu- 

 sion of one element into another. 



Thus the relatively slow rate of diffusion 

 of platinum, as compared with gold, points 

 to its having a more complex molecule than 

 the latter. 



PART II. — DIFFUSION OF SOLID METALS. 



The second part of the paper is devoted 

 to the consideration of the diffusion of solid 

 metals. Much of the evidence is historical, 

 for there has long been a prevalent belief 

 that diffusion can take place in solids, and 

 the practice in conducting impoi-tant in- 

 dustrial operations supports this view. In 

 this connection the author cites two truly 

 venerable ' cementation ' processes. The ob- 

 ject in the first of these is the removal of 

 silver from a solid gold-silver alloy; while 

 the second is employed in steel-making by 

 the carburisation of solid iron. In both oi 

 these processes, however, a gas may inter- 



vene, though the carburisation of iron by 

 the diamond, which had been effected in 

 vacuo by the author, suggests that if a gas 

 does intervene in the latter case, its quan- 

 tity must be very minute. In connection 

 with the mobility of various elements in 

 iron, the work of Colson, of Osmond and of 

 Moissan is especially referred to. 



The author points out that in 1820 Fara- 

 day and Stodart showed that platinum will 

 alloy with steel at a temperature at which 

 even the steel is not melted, and they ex- 

 press their interest in the formation of al- 

 loys by cementation, that is, bj' the union of 

 solid metals. 



The remarkable view expressed by Gra- 

 ham in 1863, that the " three conditions of 

 matter (liquid, solid, gaseous) probably 

 always exist in every liquid or solid sub- 

 stance, but that one predominates over the 

 other," is shown to have afforded ground 

 for the anticipation that metals would dif- 

 fuse into each other at temperatures far be- 

 low their melting points. Eeference is then 

 made to the important work by Spring in 

 1886 on the lead-tin alloys, which retained 

 a certain amount of molecular activity after^ 

 they had become solid, and special impor- 

 tance is attached to the proof afforded by 

 Spring, that alloys may be formed either by 

 the strong compression of the finely divided 

 constituent metals at the ordinary tempera- 

 ture (1882) or (1894) by the union of solid 

 masses of metal compressed together at tem- 

 peratures which varied from 180° in the 

 case of lead and tin to 400° in the case of 

 copper and zinc ; tin melting at 227° and 

 zinc at 415°. 



The evidence as to the volatilization of 

 solid metals is then traced, and allusion is 

 made to the expression of Kobert Boyle's 

 belief, that even such solid bodies as glass 

 and gold might respectively ' have their lit- 

 tle atmospheres and might in time lose 

 their weight.' 



Merget's experiment on the evaporation 



