180 TEAR-BOOK OF FACTS. 



the tube in successive operations, each lasting two hours. In none 

 of these cases was there any true cementation. 



With ammonia it was different : after two hours' heating, the bar 

 was immediately tempered and hammered, and again tempered, and 

 then exhibited a regular and beautiful cementation of y^- inch in 

 depth. This was attributable to the action of ammonia on carbon, 

 forming at this temperature cyanide of ammonium, which gives up 

 carbon to the iron and forms steel. A direct experiment was made, 

 omitting the charcoal, and heating an iron bar placed in a porcelain 

 tube to redness in a current of gaseous cyanide of ammonium. 

 After two hours' heating, the bar was treated as before, and was 

 found quite cemented, especially at the end nearest the place at 

 which the gas entered. 



It seemed probable that this property of cementation was not 

 confined to cyanide of ammonium, but was shared by other alkaline 

 cyanides; the cementation by means of yellow prussiate of potash 

 is probably of this kind. To decide this point experimentally, the 

 bar was placed in the tube surrounded by charcoal impregnated with 

 carbonate of potass, and heated to redness in a current of air. 

 Under these circumstances, as is well known, cyanide of potassium 

 is formed. After two hours the bar was found cemented in a mag- 

 nificent manner to a depth of ^\ inch. Soda, baryta, and strontia 

 act in the same way, but this is not the case with lime. This doubt- 

 less arises from the fact (which was proved by experiment) that it 

 does not form cyanide of calcium when heated to redness in a current 

 of cyanide of ammonium. 



The action of the various receipts for cementation may be ex- 

 plained by the formation of cyanides. It will be found that in all 

 cases they contain the elements of the formation of alkaline cya- 

 nides. — Comptes Jicndus ; Philosophical Magazine, No. 13C. 



ON THE DIFFERENCE IN SIZE OF MEDALS OF DIFFERENT METALS 

 OU I'M NED BY STAMPING, AND BY CASTING IN THE SAME MOULD. 

 BY 11. W. DOVB. 



Baudbikont lias found {Ann. de Chim. et de Phys. vol. lx. p. 78) 

 that wires of different metals drawn through the sam • press are not 

 all of the same thickness ; for they are of different degrees of elas- 

 ticity, and after being drawn through the press they expand t" dif- 

 ferent amounts. This expansion is proved by the fact that, witli 

 the exception of gold wire, no wire can be drawn throngb the Bame 

 aperture through wnioh it has been pressed. Silver requires the 

 Icnst force, but the expansion caused by elasticity continues tor seve- 

 ral weeks. 



It appeared probable that in stamping medals something similar 

 would piwail, and that medals of different metals stamped in the 

 lie would be different in sise. Tins is most readily seen in 

 those medals in which the impression is symmetrically arranged in 

 referenoa to the edge, as is tin- oase with the medals of the French 

 Exhibition, in which the coats of arms encircle the French eagle in 



