June 5, 1896.] 



SCIENCE. 



829 



of frozen mercury is quoted in relation to 

 Gay-Lussac's well-known discovery that 

 the vapors emitted by ice and water both 

 at 0° C are of exactly equal tension. 



Demargay's experiments on the volatiliza- 

 tion of metals in vacuo at comparatively low 

 temperatures is connected with the evidence 

 afforded by Spring ( 1 894 ) , that the interpen- 

 etration of the two metals at a temperature 

 below the melting point of the more fusible 

 of the two is preceded by volatilization. 



The author then points out that, interest- 

 ing as the results of the earlier experiments 

 are, as affording evidence of molecular in- 

 terpenetration, they do not, for the purpose 

 of measuring diffusivity, come within the 

 prevailing conditions in the ordinary dif- 

 fusion of liquids, in which the diffusing siib- 

 stance is usually in the presence of a large 

 excess of the solvent, a condition which has 

 been fully maintained in the experiments 

 on the diffusion of liquid metals described 

 in the first part of the paper. Van't Hoff 

 has made it highly probable that the 

 osmotic pressure of substances existing in 

 a solid solution is analogous to that in liquid 

 solutions and obeys the same laws ; and it 

 is probable that the behavior of a solid 

 mixture, like that of a liquid mixture, 

 would be greatly simplified if the solid solu- 

 tion were very dilute. 



The author proceeds to describe his own 

 experiments on the diffusion of solid metals. 

 They are of the same nature as in the case 

 of fluid metals, except that the gold, which 

 is the metal chosen for examination, was 

 placed at the bottom of a solid cylinder of 

 lead instead of a fluid one. 



In the first series of experiments, cylin- 

 ders of lead, 70 mm. long, with either gold, 

 or a rich alloy of gold and lead at their 

 base, were maintained at a temperature of 

 251° (which is 75° below the melting point 

 of lead) for thirty-one days. At the end 

 of this period the solid lead was cut into 

 sections, and the amount of gold which had 



diffused into each of them was determined 

 in the usual way. Other experiments fol- 

 low, in which the lead was maintained at 

 200° and at various lower temperatures 

 down to that of the laboratory. The fol- 

 lowing are the results : 



k. 



Diffusivity of gold in fluid lead at 550° 3.19 



" solid " 251° 0.03 



" " " 200° 0.007 



" " " 165° 0.004 



" " " 100° 0.00002 



The experiments at the ordinary temper- 

 ature are still in progress, but there is evi- 

 dence that slow diffusion of gold in lead 

 occurs at the ordinary temperature. The 

 author points out that if clean surfaces of 

 lead and gold are held together in vacuo at 

 a temperature of only 40° for four days 

 they will unite firmly, and can only be sep- 

 arated by the application of a load equal to 

 one-third of the breaking strain of lead it- 

 self. 



The author thinks it will be considered 

 remarkable that gold placed at the bottom 

 of a cylinder of lead, 70 mm. long (which 

 is to all appearance solid), will have dif- 

 fused to the tip in notable quantities at the 

 end of three days. He points out that at 

 100° the diffusivity of gold in solid lead can 

 be readily measured, though its diffusivity is 

 only xoTTo of that in fluid lead at a tem- 

 perature of 500°. He also states that ex- 

 periments which are still in progress show 

 that the diffusivity of solid gold in solid 

 silver or copper at 800° is of the same 

 order as that of gold in solid lead at 100°. 



He concludes by warmly thanking Mr. 

 A. Stansfield, B.Sc, who assisted him in all 

 but the earlier portion of the work, and by 

 expressing the hope that the experiments 

 described in the paper will show that the 

 diffusion can readily be measured in solid 

 metals, and that they will carry one step 

 further the work of Graham. 



