ORGANIC ANALYSIS. 707 



perfectly fixed at a low red heat. The chromate of lead is not 

 in the slightest degree hygroscopic, and is likely to be preferred 

 to oxide of copper, where it is desirable to determine the 

 proportion of hydrogen with extreme accuracy. 



Notwithstanding the great value of the analytical results of this 

 method, and the agreement almost perfect in repetitions of the 

 same analysis, there can belittle doubt that the method itself is 

 not absolutely exact. From the rigorous examination to which 

 the combustion process has lately been submitted by M. Dumas, 

 it appears to give less than the true quantity of carbon. The 

 loss of carbon is ascribed to several causes : some is deposited 

 here and there in the tubes and for want of oxygen not burned ; 

 the reduced copper is partly converted into carburet of copper ; 

 the liquid potash allows a portion of the carbonic acid to escape, 

 and lastly the air which is drawn through the apparatus takes 

 up some water from the same potash and diminishes its weight. 

 This loss of carbon was hitherto concealed by carbonic acid 

 being allowed to contain more carbon than it really has, so 

 that the carbon lost in the process was made up in the calcu- 

 lation; and the formulae deduced from analyses are only 

 true from the accidental compensation of these two errors. 

 Dumas reduces the proportion of carbon in carbonic acid from 

 27.67 to 27.27 per cent, and obtains for the atom of that 

 element the number 75, instead of 76.4. This important result 

 he has deduced by collecting and weighing the carbonic acid pro- 

 duced by the combustion of a known weight of pure charcoal, in 

 the forms of graphite and the diamond.* Drs. Marchand and 

 Erdman of Berlin have repeated these analyses with the same 

 results; it is now indeed generally allowed that the atomic 

 weight of carbon of Berzelius is too high, but chemists are 

 not yet agreed as to the amouut of reduction to be made. 

 MM. Redtenbacher and Liebig conclude that the atomic 

 weight of carbon is the intermediate number 75.854, from an 

 elaborate series of experiments undertaken to determine the 

 point, in which the proportion of silver in the acetate, malate, 

 racemate and tartrate of that base was ascertained with great 

 accuracy ; the atomic weight of silver, respecting which there 

 is little uncertainty, being taken at 1351.6. It has also been 



* MM. Dumas and Stass, Annalcs dc Chimie et de Physique, 3me. S^ric, 

 tome 1, p. 5. (1841). 



