CHARLES DARWIN 311 



In the finest elements of structure, right down to the 

 atoms, progress was made in the investigation of living 

 matter by the aid of the chemical knowledge obtained by 

 Scheele, Dalton, Davy and Berzelius. New substances 

 with characteristic properties were continually being dis- 

 covered and investigated; they were taken from plants and 

 animals, separated, and prepared in a pure state. Scheele 

 himself had already made a good beginning, when he pre- 

 pared in a pure form such substances as tartaric acid, citric 

 acid, and malic acid, and found out how to distinguish them 

 one from another. These substances were then analysed, 

 and no other elements were found in them than those already 

 known from non-living nature, the chief being always car- 

 bon, and along with it almost always only hydrogen, oxygen 

 and sometimes nitrogen; otherwise surprisingly few further 

 elements. In view of this uniformity, it became all the 

 more important to discover the difTerences in their quanti- 

 tative composition, for which purpose Berzelius, and 

 twenty-five years later (1837) in a more exact manner, 

 Liebig, developed suitable methods particularly adapted to 

 the case of these few elements. 



Berzelius was already able to show that in organic com- 

 pounds, as the substances occurring in plants and animals 

 were early called, fixed proportions by weight of the ele- 

 mentary components also hold good, and can be expressed 

 in the numerical relationships of Dalton's atoms. The 

 numbers of atoms which had to be regarded as combined 

 with one another in these cases, were often fairly large, and 

 hence difficult to determine with accuracy; but gradually the 

 recognition of the recurrence of certain smaller groups of 

 atoms (radicles) as a common constituent of the larger 

 groups, was of great assistance. The structure of the latter 

 could thus be determined without dependence upon quanti- 

 tative analysis alone, and without the determination of mole- 

 cular weight, for which purpose the methods given by 



