42 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



is carried out in practice by heating a weighed quantity of the 



substance to be analysed in a small porcelain boat placed in 



a tube about a yard long (Fig. 9) filled with 



granulated oxide of copper, and through which a 



current of oxygen or air can be passed. The 



whole tube is heated in a furnace, and any 



partially burned vapour of the substance which 



escapes direct combustion is fimally oxidised by 



passing over the red-hot copper oxide. The 



; water is retained in a tube containing calcium 



' chloride, which readily absorbs moisture, and 



1 the CO2 is retained in specially devised bulbs 



S fUled with caustic potash, which are weighed 



; before and after the analysis. This process is 



. known as combustion analysis and is regularly 



\ employed in laboratories devoted to organic 



I chemistry. Special methods, of course, are made 



: use of in the determination of elements other 



j than carbon and hydrogen, e.g., nitrogen, phos- 



' phorus, or sulphur. Oxygen is usually deter- 



' mined by difference, i.e., by deducting the 



j weights of all the other elements present from 



the weight of the substance originally taken, 



I when the remainder, if any, is assumed to be 



oxygen. The determination of the percentage 



composition of the substance from combustion 



analysis wiU be made clear by the following 



example : — 



0-2 grm. of a substance vielded on analysis 0-290 grm. 



CO2 and 012 grm. H2O. 



Now in every 44 parts COj there are 12 parts C, 

 therefore in 029 grm. COj there will be : — 



0-29 X 12 

 44 



= 0-079 parts C 



