396 Professor W. Chandler Roberts- A listen [March 26, 



and liquid matter in a greater or less degree. The colloid or jelly- 

 like body does present a certain amount of resistance to change of 

 BhajDC. Such a substance is well imitated by a skin of thin india- 

 rubber filled with water. Another illustration is probably afforded 

 by iron and other substances which soften under heat, and may be 

 sup230sed to assume, at the same time, a colloid condition. Lastly, 

 there is the gaseous condition of matter, with which we have but 

 little to do at present. 



We are in the habit of regarding metals as typical solids. I hope 

 to trace this evening the analogies of their behaviour under certain 

 conditions with that of fluids, and the following list shows the order 

 in which I propose to group the proj)erties common to fluids and 

 solid metals : 



1. Rejection of impurities on solidification. 



2. Snr fusion. 



3. Flow under pressure. 



4. Changes due to compression. 



5. Absorption of gases. 



6. „ liquids. 



7. Diffusion. 



8. Vaporisation. 



9. Surface tension. 



The transition from the liquid to the solid state is marked by the 

 same phenomena in the case of many metals, as is observed in certain 

 fluids, and I must dwell on this very briefly as leading up to the 

 relations between solid metals and fluids, which come more definitely 

 within the title of the lecture. 



Water on passing from the liquid to the solid state undergoes a 

 partial purification, the ice first formed being sensibly more free from 

 colouring matter or suspended particles than the water from which it 

 separates. 



Many metals on freezing similarly eject impurities. In the case 

 of alloys, saturated solutions, of one metal in another, appear to be 

 formed, and excess of metal ejected, a fact which is being studied 

 with much care by my colleague, Dr. Guthrie. The prominent facts 

 are perhaps best illustrated by reference to a frozen mixture of 

 copper, antimony, and lead. The results of some experiments con- 

 ducted in my laboratory by my assistant, Dr. E. J. Ball, show that 

 when a molten mixture of these metals is solidified, a definite atomic 

 alloy of copper and antimony, which possesses a beautiful violet tint, 

 first forms, and, after saturating itself with lead, up to a certain 

 point, it ejects the rest of the lead, driving it to the centre of the 

 mass so as to form a sharp line of demarcation, as is shown in the 

 engraving. Fig. 1, the outer circle of which represents violet, and the 

 inner grey, presenting a direct analogy to the ice, which is compara- 

 tively colourless, first forming from coloured water. Then there is 

 another remarkable analogy between the freezing of certain fluids 

 iind the solidification of some metals. Water may, as is well known, 



