28 Biological Chemistry. 



flasks with ground stoppers made to fit tightly, which are 

 inserted during the weighings. 



THE ESTIMATION OF CARBON AND HYDROGEN. 



In the older process of Liebig, a weighed amount of 

 the substance is introduced into a long glass tube in a 

 porcelain boat,* behind a long layer of granular copper 

 oxide, and a shorter layer of the oxide (generally an 

 oxidized copper spiral) is placed behind it. The tube is 

 then introduced into a special form of furnace, and the 

 foremost and hindmost parts of the tube are heated to dull 

 redness, whilst a current of air, carefully freed from carbon 

 dioxide and moisture, is led at a uniform rate through 

 a suitably constructed purifying apparatus into the back 

 of the tube. Into the front part of the tube is in- 

 serted a U tube containing calcium chloride or pumice 

 soaked in sulphuric acid, which absorbs the moisture pro- 

 duced by the combustion, and this is joined on in series 

 to two other tubes containing soda-lime, or to bulbs 

 containing a strong solution of caustic potash to absorb 

 the carbon dioxide. These various tubes or bulbs are 

 weighed both before and after the combustion, and their 

 gain in weight indicates the amount of water and carbon 

 dioxide formed by the oxidation of the substance. As 

 soon as the copper oxide before and behind the substance 

 has attained a dull red heat, the substance itself may be 

 heated ; it then gradually decomposes and chars. The 

 temperature is then gradually raised, and the final stages 

 of the combustion are now usually completed in a current 

 of oxygen instead of air. Air is finally led through the 

 tube until all the pure oxygen is displaced, and all the 

 products of combustion have been passed through the 



* Or in a small glass bulb if the substance is a liquid* 



