CHEMISTRY AND ITS DEVELOPMENT 229 



composition of substances in the simplest and clearest pos- 

 sible manner. In the forties Laurent and Gerhardt became 

 convinced that the progress of knowledge in organic chem- 

 istry was seriously impeded by the lack of a consistent 

 system of atomic weights. Their researches soon led them 

 to distinguish clearly between the atoms and molecules of 

 elements, and to grasp the full value of Avogadro's prin- 

 ciple for determining the relative weights of molecules. 

 With the aid of this principle, Gerhardt found the true 

 atomic weights of the elements; and, in the latter part of 

 the fifties, his pupil Cannizzaro demonstrated clearly the 

 consistency of the principle with all known facts. Thus 

 was paved the way for the doctrine of valency. A few 

 years later (in 1869) Mendeleeff and Lothar Meyer estab- 

 lished a remarkable connection between the properties of 

 the elements and their atomic weights (see Periodic Law) 1 

 and thus the correctness of the latter was confirmed in a 

 very striking manner. 



The further progress of general chemistry has been 

 mainly in connection with the various subdivisions of 

 physical chemistry, brief historical accounts of which may 

 be found under the following headings : Reaction, Solution, 

 Dissociation, Thermo-chemistry, Electro-chemistry and 

 Laboratory. 1 



*New International Encyclopaedia. 



