On the Diffusion of Gold in Solid Lead. 101 



but in the later hours it produced an increase of from 4'3 to 11*0 per 

 cent. The reaction of the organism to a constant environmental con- 

 dition was thus a variable one. This is probably explicable by the fact 

 that the temperatures necessary to kill the organisms, and presumably 

 also those which cause an unfavourable effect on growth, rise steadily 

 during development. Thus the death temperature is about 28'5 for 

 unsegmented ova, 34 for blastulse, and 40 for plutei. 



The impregnated ova Avere also found to be much more sensitive to 

 changes in the salinity of the water during the early stages of 

 development than during the later ones. 



" On the Diffusion of Gold in Solid Lead at the Ordinary Tempe- 

 rature." By Sir W. ROBERTS-AUSTEN, K.C.B., F.K.S., Professor 

 of Metallurgy, Royal College of Science. Eeceived April 5, 

 Eead May 10, 1900. 



In the Bakerian Lecture, " On the Diffusion of Metals,"* delivered 

 in 1896, evidence was given to show that gold placed at the base of a 

 column of fluid lead 16 cm. high, maintained at a mean temperature of 

 492, or 166 above the melting point of lead, diffuses to the top of the 

 column in an appreciable amount in a single day, the diffusivity ex- 

 pressed in centimetre-day units being 3'0. If the lead be heated, say 

 to 251, or 75 below the melting point of the metal, diffusion takes 

 place at a much slower rate ; it may still be readily measured, though 

 the diffusivity is only 0'023 in centimetre-day units. In the experi- 

 ments on diffusion in solid lead, the latter metal was prepared with 

 great care, and possessed a high degree of purity. The method of 

 preparation consisted in the reduction of carefully purified carbonate 

 of lead by cyanide of potassium, the reduced metal being cast in 

 carbon moulds. 



It became evident that at the ordinary temperature the rate of 

 diffusion of solid gold in solid lead must be very slow, and I stated in 

 the Bakerian Lecture that cylinders of lead had been set aside with 

 discs of gold affixed to their bases, in order that, after a sufficient lapse 

 of time, the diffusion occurring at the ordinary temperature might be 

 measured. By the month of March in the present year, four years 

 had elapsed since the experiment began, and the time appeared to be 

 sufficiently long to justify the attempt to ascertain how far the gold 

 had diffused. In starting the experiments the bases of the lead 

 cylinders were carefully brought to a smooth surface, and the discs of 

 pure gold were specially cleaned, the discs of gold being held against 



* Delivered February 20, 1896. ' Phil. Trans.,' A, vol. 187 (1896) pp. 383415. 

 VOL. LXVII. I 



