464 Mr. J. Dewar on the Specific Heat of 



zation^ we have 36,000 units left to be expended in raising the 

 carbon to its boiling-point. Despretz has shown that carbon 

 does not liquefy in the ordinary sense, but passes at once into 

 the form of vapour, so that we may consider it solid up to its 

 point of vaporization. 



It now only remains to ascertain the rate at which the specific 

 heat of carbon varies with the temperature; and it will be shown 

 in the sequel that the mean specific heat between 0°and 2000° C. 

 is 0*42. The amount of heat required to raise the 12 grammes 

 of carbon 1° C. is therefore 0*42 X 12 = 5*04 ; and the approxi- 

 mate boiling-point is thus 36,000--5-0 = 7200° C. If we give 

 the first oxide 16,000 units in excess of the second, the boiling- 

 point would then only reach 10,000^ C. ; and it would be diffi- 

 cult to make it any higher by more favourable suppositions based 

 on analogy. 



The first series of experiments on the mean specific heat of 

 carbon at high temperatures were made between 1040° C. and 

 20° C. For this purpose a large plumbago crucible, holding 

 about 30 lbs. of zinc, was heated up to the boihng-point by means 

 of a smithes forge and kept continuously boiling by means of a 

 regulated blast. Into this bath v/rought-iron tubes about half 

 an inch in diameter were inserted, with ground iron stoppers for 

 the purpose of holding the pieces of carbon. The part of the 

 tubes above the surface of the zinc have each a short screw turned 

 on the outside in order to attach a plate of sheet iron 8 inches 

 in diameter with rapidity before the tube is removed. The 

 object of the plate is to prevent any particle of zinc adhering to 

 the tube being thrown, in the sudden transference along with 

 the carbon, into the calorimeter, and to prevent radiation. The 

 tubes remain in the bath about twenty minutes; the plate of 

 iron is then screwed on; the -smith catches the iron tube near the 

 surface of the zinc with a pair of tongs while the iron stopper 

 is removed ; and the carbon is instantly transferred to the calori- 

 meter. The pieces of carbon are not broken or disintegrated by 

 the action of the water ; nor is there any steam generated. After 

 the experiments they were dried and weighed. The experiments 

 have always been made with the purest French gas-retort car- 

 bon, selecting pieces as free as possible from ash. The calori- 

 meter was carefully surrounded with three cylindrical rings, 

 of which the two outer ones were filled with water in order to 

 keep the temperature of the interior constant. The following 

 Table contains the experimental results of a few observations 

 executed in the above manner : — 



