viii Preface to the New Edition 



illustrative types of necessarily 'natural' substances, classified 

 on the basis of ascertained chemical variations, and of deriva- 

 tives resulting from well-known processes of synthetic addition, 

 or of more or less destructive resolutions, chiefly by processes 

 of hydrolysis and oxidation. These relationships set out in 

 the logical order of the systematic science are of permanent 

 significance, and until we have an accepted theory of the state 

 of matter in these organic and organised products of the plant 

 world it would be superfluous to devise any other plan. Its 

 basis is that of an orderly empiricism, and in absence of any 

 criterion of actual molecular constitution especial prominence 

 is given to the weight statistics of the reactions described. Any 

 molecular units in this region must be more or less arbitrarily 

 fixed ; a unit of 100 is justified by convenience, and percentage 

 results are as exact as molecular proportions. In the case of 

 certain of the esters of the typical cellulose, these appear to 

 have a simple relationship to the parent substance, and the use 

 of molecular formulae therefore justified. But even in such 

 cases the reactions of esterification appear to be continuous, 

 and therefore molecular formulas have only a certain value as 

 applied to the limiting cases or end products. It is open to 

 question whether, even in the case of the nitrates, we gain 

 anything by a quasi-molecular terminology. 



The limiting nitrate being the tri-nitrate (C 6 formula), 

 representing 14*1 p.ct. N, a collodion nitrate of 11*2 

 p.ct. N is correctly described as an 80 p.ct. Nitrate 



(80 = -3- x 100). The molecular proportion is n = 2-02, 

 14*1 



and the body approximately a dinitrate. But the formula 

 C 6 H 8 O 3 .(NO 3 )2 is of no significance as identifying a chemical 

 individual. The specialist could produce a series of nitrates 

 of this formula, differing widely in characteristics, e.g.^ solu* 



