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constructed with tho old system of atomic weights. At the same time it appears 

 desirable also to introduce the use of the modern atomic weights ; and, therefore, in 

 almost every chemical expression in this work both tho old and the new formulae are 

 given the latter being printed, for sake of distinction, in a thick black type. 



In the folio wing comparative Table tho first column of figures gives (approximately) 

 the old combining weights of the elements, - whilst the second column gives the 

 modern atomic weights, as employed by the most advanced chemists of the present 

 day. A glance at this Table is therefore sufficient to show which of the elements have 

 retained their old atomic weights, and which have had them doubled. Moreover, 

 it is easy from these data to translate any of tho- formula) on the old system into 

 formulae constructed with the recent values of the atomic weights. For example, 

 in our article ALCOHOL, that compound is said to be composed of C 4 H 6 O 2 ; but 

 reference to the Table shows the atomic weights of carbon and of oxygen have been 

 doubled, whilst that of hydrogen remains unaltered. It is, therefore, obviously 

 necessary to halve the number of atoms of carbon and of oxygen in tho old formula, 

 and consequently the symbolic expression for alcohol, instead of being C 4 II" O v , 

 becomes on the modern system C 3 H 6 O. 



