390 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



Organic analyses of Gay-Lussac and Thenard (1810), 



The foundations of organic analysis were laid by Lavoisier 

 in the two papers that have already been quoted (pp. 145- 

 147) ; but it was not until many years later that simple and 

 accurate methods became generally available. Some of the 

 earlier workers, including Berth ollet, relied upon decompos- 

 ing the organic compounds by heat into charcoal, which 

 could be weighed, and gaseous products which could be 

 analysed in the eudiometer. 



In 1810 a general method of analysis was worked out by 

 Gay-Lussac and Thenard (Phy si co- Chemical Researches, 

 1811, II. 265-350) and applied to four substances contain- 

 ing nitrogen and fifteen substances free from nitrogen. The 

 substance was mixed with a known weight of potassium 

 chlorate, made into pellets and burnt by dropping into a hot 

 tube (Fig. 49). The gases produced by burning the pellets 

 (oxygen, carbonic anhydride, and nitrogen, if present) were 

 collected and analysed. From the volume of carbonic 

 anhydride the weight of carbon in the substance could be 

 calculated ; a similar calculation gave the weight of nitrogen. 

 Blank experiments showed what volume of oxygen was set 

 free by igniting the chlorate alone. In several cases the 

 deficiency of oxygen was exactly equal to that due to the 

 formation of carbonic anhydride ; any part of the weight 

 of the substance not accounted for after adding together the 

 carbon and nitrogen, together with other easily recognised 

 elements, such as chlorine, phosphorus and the metals, was 

 then regarded as water. A greater or less deficiency of 

 oxygen indicated the presence of an excess of hydrogen or 

 of oxygen, above the proportions required to produce water. 



The analyses of Gay-Lussac and Thenard revealed the 

 remarkable fact that six of the substances analysed (sugar, 

 gum, starch, milk-sugar, oak-wood, and beech-wood) con- 

 tained carbon united with hydrogen and oxygen in just the 

 proportions in which they are required to form water. Such 

 compounds are now known as CARBOHYDRATES. These six 



