302 DISCOURSE ON THE STUDY 



commenced (in 1786) when Lavoisier, by a series of 

 memorable experiments, extinguished for ever this 

 error, and placed chemistry in the rank of one of the 

 exact sciences, a science of number, weight, and 

 measure. From that epoch to the present day it has 

 constantly advanced with an accelerated progress, 

 and at this moment may be regarded as more pro- 

 gressive than ever. The principal features in this 

 progress may be comprised under the following 

 general heads : 



1. The discovery of the proximate, if not the 



ultimate, elements of all bodies, and the en- 

 largement of the list of known elements to 

 its present extent of between fifty and sixty 

 substances. 



2. The developement of the doctrine of latent 



heat by Black, with its train of important 

 consequences, including the scientific theory 

 of the steam-engine. 



3. The establishment of Wenzel's law of definite 



proportions on his own experiments, and 

 those of Richter, a discovery subsequently 

 merged in the more general wording and 

 better development of Dalton's atomic 

 theory. 



4. The precise determination of the atomic weights 



of the different chemical elements, mainly 

 due to the astonishing industry of Berzelius, 

 and his unrivalled command of chemical re- 

 sources, as well as to the researches of the 

 other chemists of the Swedish and German 

 school. 



5. The assimilation of gases and vapours, by which 



