620 



NATURE 



[October 27, 1898 



of great interest, that a very minute quantity of a solid 

 will cause a mass of the same substance to pass from 

 the surfused to the solid state. His work, moreover, has 

 led him to distinguish between the meta-stable, or 

 ordinary condition in which surfusion takes place, and 

 the labile condition which occurs at temperatures much 

 below the melting point. Ostwald's paper, and one by 

 M. Brillouin (^««. de Chim. et de Phys., 1898, vol. xiii. 

 p. 264), on the theory of complete and pasty fusion, led 

 the author to offer the Royal Society the results of his 

 own experiments on the surfusion of metals. 



Metals do not appear to have been examined from the 

 point of view of surfusion until the year 1880, when some 

 excellent experiments on the surfusion of gold were made 

 by the late Dr. A. D. van Riemsdijk {Ann. de Chim. et 

 de Phys,, 1880, vol. xx. p. 66), by whose early death, 

 which occurred last year, Holland has lost a skilful 

 physicist. He pointed out that :— 



" Faraday, in his memoir on regelation, published in 

 1858, stated that acetic acid, sulphur, phosphorus, many 

 metals and many solutions, may be cooled below the 

 freezing temperature prior to solidification of the first 

 portions " (" Experimental Researches in Chemistry and 

 Physics," p. 379). On the other hand, in their treatises 

 on physics, Danguin (vol. i., 1855, p. 892) and Jamin (vol. 

 i., 1859, p. 105) mention tin as the only metal which is 

 capable of remaining liquid at a temperature 2-5° below 

 the true solidifying point of the metal. 



Van Riemsdijk's contribution to the subject of sur- 

 fusion of metals consisted in showing that the well-known 

 phenomenon of eclair, the brilliant flash of light which 

 often attends the solidification of the metal in the ordinary 

 assay of gold, is really due to surfusion. He also pointed 

 out that surfusion could be either stimulated or hindered 

 by suitably modifying the conditions, but he made no 

 attempt at thermal measurements. It was not until ten 

 years after van Riemsdijk's work that the recording 

 pyrometer, which the author submitted to the Royal 

 Society in 1891 {Proc. Roy. Soc., 1891, vol. xlix, p. 347), 

 enabled such measurements to be readily effected. 



After a brief description of this appliance, the nature 

 of which is now well known, it is stated that the freezing 

 point of a metal, or the initial freezing point of an alloy, 

 may be represented by one or other of three typical 

 curves. Two of these are shown in the accompanying 

 figures, which indicate the nature of the curves, traced 

 by the recording pyrometer. Fig. i shows the freezing 

 point curve of a pure metal, the horizontal portion, a d, 

 mdicating the actual solidification of the mass, the 

 sharpness of the angles at a and b attesting the purity 

 of the metal. The initial freezing point of most alloys 

 would resemble Fig. i in having the corner a sharp, 

 while the point b is generally rounded off. 



The third type of curve, which may be a modification 

 of the other two types, indicates the occurrence of sur- 

 fusion, the bend at a. Fig. 2, showing the amount of sur- 

 fusion which was observed. The author has detected pro- 

 nounced cases of surfusion not only in gold, but in copper, 

 NO. 15 I 3, VOL. 58] 



bismuth, antimony, lead, and tin. Surfusion, moreover, 

 is not confined to pure metals, and he showed in 1893, 

 that the eutectic alloy in the bismuth-copper series 

 presents a marked case of surfusion. In order to study 

 surfusion, it is necessary to make the galvanometer (to 

 which the thermo-junction is attached) very sensitive. 

 The method of effecting this is described, the thermo- 

 junction itself being in all cases suitably protected and 

 placed in the cooling mass of metal or alloy. A curve, 

 traced by the aid of such a sensitive method, if it repre- 

 sents the surfusion of a metal or an alloy, does not merely 

 show a slight depression as in the case of pure gold 

 shown at a, Fig. 2 : the slight depression becomes a deep 

 dip. It is, in fact, possible by the methods described by 

 the author to ascertain what takes place during the 

 surfusion of an alloy, and the results are shown in two 

 plates appended to the paper. From these plates one 

 illustration (Fig. 3) has been selected. It is the autographic 

 representation of the surfusion of an alloy of 64 parts of 



Fig. 3.-64 tin, 36 lead. 



tin and 36 parts of lead. The line a b represents the 

 surfusion of the mass which, as the scale shows, fell 

 10 degrees below its true point of solidification before 

 it actually became solid. The solidification of the mass 

 is recorded by the horizontal line c. This autographic 

 record also shows that something happened during 

 surfusion, for there are points at d and e. These proved 

 to be due to the falling out of lead at d, and to its having 

 to be remelted at e. The entire mass then became solid. 



Experiments such as the one described have enabled the 

 author to trace the crossing of solubility curves of certain 

 metals in each other in the same way as had previously 

 been effected in the case of salts by H. le Chatelier and 

 by Dahms. 



This crossing of the solubility curve of lead and tin is 

 shown in Fig. 4, but for a description of it reference must 

 be made to the original paper. 



The first experimental evidence as to the identity of 

 the behaviour of saline solutions and metallic alloys as 

 regards selective surfusion, has thus been afforded by 



