492 Mr. William Crockes [June 11, 



means of obtaining high temperatures. Thanks to the success of 

 Professor Moissan, whose name will always be associated with the 

 artificial production of diamonds, we are able to-day to manufacture 

 diamonds in our laboratories — minutely microscopic, it is true — all 

 the same veritable fliamonds, with crystalline form and appearance, 

 colour, hardness, and action on light the same as the natural gem. 



Until recent years carbon was considered absolutely non-volatile 

 and infusible ; but the enormous temperatures at the disposal of ex- 

 perimentalists — by the introduction of electricity — show that, instead 

 of breaking rules, carbon obeys the same laws that govern other bodies. 

 It volatilises at the ordinary pressure at a temperature of about 3600°O., 

 and passes from the solid to the gaseous state without liquefying. 

 It has been found that other bodies which volatilise without liquefying 

 at the ordinary pressure will easily liquefy if pressure is added to 

 temperature. Thus, arsenic liquefies under the action of heat if the 

 pressure is increased ; it naturally follows that if | along with the 

 requisite temperature sufficient pressure is applied, liquefaction of 

 carbon will be likely to take place, when on cooling it will crystallise. 

 Put carbon at high temperatures is a most energetic chemical agent, 

 and if it can get hold of oxygen from the atmosphere or any compound 

 containing it, it will oxidise and fly off in the form of carbonic acid. 

 Heat and pressure therefore are of no avail unless the carbon can be 

 kept inert. 



It has long been known that iron when melted dissolves carbon, 

 and on cooling liberates it in the form of graphite. Moissan dis- 

 covered that several other metals have similar properties, especially 

 silver; but iron is the best solvent for carbon. The quantity of 

 carbon entering into solution increases with the temperature, and on 

 cooling in ordinary circumstances it is largely deposited as crystalline 

 graphite. 



Professor Dewar has made a calculation as to the Critical Pressure 

 of carbon — that is, the lowest pressure at which carbon can be got to 

 assume the liquid state at its critical tcmjierature, that is the highest 

 temperature at which liquefaction is possible. He starts from the 

 vaporising or boiling point of carbon, which, from the experiments of 

 Yiolle and others on tlie electric arc, is about 3600° C, or 3874° 

 Absolute. The critical point of a substance on the average is 1 * 5 

 times its absolute boiling point. Therefore the critical point of 

 carbon is 5811° Ab., or, say, 5800° Ab. But the absolute critical 

 temperature divided by the critical pressure is for elements never 

 less than 2 • 5. Then — 



6800° A. ^ . -p^ 5800° A. ^qoh . i. 

 — =r— — = 2 • 5, or PCr = — -^ — , or 2d20 atmospheres. 

 PCr 2 • 5 



The result is that the critical pressure of carbon is about 2300 

 atmospheres, or say 15 tons on the square inch. The highest critical 

 pressure recorded is that of water, amounting to 195 atmospheres, 



