vi THE COMPOSITION OF FIXED AIR 99 



Diamond and graphite. Newton had suspected (Opticks, 

 1704, II. 75) that the diamond, with its high refractive 

 power, might be a combustible substance. This idea was 

 confirmed by several notable experiments in which the 

 combustion was effected both by powerful burning-glasses 

 and by means of furnaces. But as the experiments were 

 always made in the open, no idea could be formed of the 

 nature of the products ; indeed, it was generally believed 

 that the diamonds had merely been vaporised without 

 burning at all. 



To Lavoisier belongs the credit of having shown, in 1772, 

 in conjunction with Macquer and Cadet, that if air be com- 

 pletely excluded, the diamond remains unaltered at the 

 highest temperature of the furnace ; the burning of the 

 diamond is therefore a true combustion. In order to deter- 

 mine the nature of the products of combustion, Lavoisier 

 employed a very large burning glass (compare Fig. 22) to heat 

 diamonds supported in air or oxygen contained in glass 

 jars inverted over water or over mercury. No water, smoke, 

 or soot was produced ; when mercury was used the volume 

 of gas was not altered, but in contact with water the volume 

 was somewhat diminished; in both cases the gas in which 

 the diamond had been burnt turned lime-water milky 

 ("Destruction of the Diamond by Fire," 1772, Works, II. 

 38-88). Charcoal behaved in just the same way as the 

 diamond. There was, therefore, no doubt that each of 

 these substances gave rise to fixed air as the sole product of 

 combustion. A few years later, in 1797, Tennant (an 

 English chemist who had already shown that carbon could 

 be recovered from fixed air by the action of phosphorus 

 vapour on red hot chalk), burned diamonds by means of 

 melted saltpetre, and showed that they gave rise to precisely 

 the same quantity of fixed air as when charcoal was used. This 

 observation was confirmed by the combustions carried out 

 many years later by Dumas and Stas (Chapter VIII, p. 150) ; 



H 2 



