462 Mr. J. Dewar on the Specific Heat of 



gap tliat I thought of making a series of experiments^ employing 

 the boiling-points of sulphur^ cadmium^ and zinc as fixed points. 

 The experiments on carbon were made before I became ac- 

 quainted with Dr. Weber^s paper, and were in reality undertaken 

 as part of a communication made to the Royal Society of Edin- 

 burgh on the 1st of April, 1872^ entitled^ ^^ On recent Estimates 

 of Solar Temperature.^^ An abstract of Dr. Weber^s excellent 

 paper appeared in the 'Chemical Journal^ for the month of July ; 

 and I am surprised he should have overlooked this fact before 

 charging the entire Chemical Section of the British Association 

 with a startling amount of ignorance regarding the progress of 

 scientific research in Germany. 



Yours respectfully, 



James Dewar. 



As carbon, of all known elementary bodies, is the most refrac- 

 tory, it would be a matter of some interest at the present time 

 to arrive at some approximate knowledge regarding its boiling- 

 point. If this could be defi.ned within reasonable limits, it 

 would be a strong argument against those enormously high tiim- 

 peratures recently attributed to the sun by Secchi and AVaterston. 



In order to acquire some idea of the highest temperatures pro- 

 duced by chemical action, suppose we calculate the hypothetical 

 maximum temperatures that could be produced during the for- 

 mation of some of the stable oxides — that is, neglecting dissocia- 

 tion and any increase in the specific heat of the product at high 

 temperatures. On this supposition it is easy to show that silicon 

 would give in oxidizing 19,500^ C.^ aluminium 15,000" C.^ and 

 magnesium about 14,600^ C. As these are the highest results 

 that can be obtained, we may conclude that direct cheniical energy 

 could not produce a temperature above 15,000" C. in the case 

 of the formation of binary compounds; and as more complicated 

 groupings are generally less stable at high temperatures, we may 

 regard this point as a maximum. It is worthy of remark that 

 the above products form a large portion of the earth^s crust; 

 and the observation of Despretz. that magnesia, when exposed to 

 the temperature of the electric arc, only became partially fused^ 

 and did not, like all the other substances experimented on, vola- 

 tilize, strongly supports the stability of this oxide at very high 

 temperatures. 



Andrews showed many years ago that, in the case of a metal 

 forming two oxides in the same physical condition, the number 

 of heat-units generated in passing from the metal to the first 

 oxide was nearly identical with those obtained in the combustion 

 of the first oxide itself; in other words^. the thermal energy is 



