CHEMISTRY OF THE NINETEENTH CENTURY. 60 1 



sciences ; and since Dalton's time, it has entered into the very texture of 

 the science, so that nearly all its generalizations are expressed in terms 

 of this theory. It has, however, been contended that since the things 

 that actually come under the chemist's observation are not atoms but 

 the relative weights of the elements in a compound, and the reactions 

 of substances under given circumstances, so the facts may be summed 

 up in general statements without introducing hypothetical ideas at all. 

 This is no doubt true ; but combinations of equivalents ef substances 

 present vague ideas to the mind compared with the more definite con- 

 ception of the atoms. These address themselves to our imaginations 

 as images of concrete things. 



Dalton conceived of his atoms as solid particles having some definite 

 shape or figure, concerning which, however, he offered no speculations. 

 But he pictured to his imagination a jar of gas as like a vessel filled 

 with small shot, the shot-corns being all of the same size. The 

 particles of the gas Dalton conceived to differ from the shot in that 

 while the shot globules are uniformly hard throughout, the gas particles 

 " are constituted of an exceedingly small central atom of solid matter, 

 which is surrounded by an atmosphere of heat of great density next 

 the atom, but gradually growing rarer, according to some power of 

 the distance." Dalton's conception was, therefore, perfectly clear. 

 There was the very small solid particle in the centre of an atmosphere 

 of the more subtle substance, which the philosophers of Dalton's time 

 called caloric. It was the repulsive force of the caloric which opposed 

 itself to the tendency of the solid particles to approach each other, 

 and the same force withstood the external pressure to which the gas 

 might be subjected. 



We have already seen that the doctrine of phlogiston was completely 

 overthrown by Lavoisier. This doctrine seemed in its time to explain 

 many phenomena of calcination, reduction, etc., and men firmly be- 

 lieved that phlogiston had a real existence. The caloric theory super- 

 seded phlogiston by better explaining a wider range of phenomena. 

 Caloric was. to almost the middle of the present century, a very clear 

 and definite conception in the minds of all scientific men. But the 

 eighth decade of the century finds caloric relegated to the same limbo 

 of things forgotten as phlogiston. The caloric theory has in its turn 

 been replaced by another doctrine of far greater scope. We have 

 already discussed this modern doctrine on page 530, so that it need 

 not be more particularly alluded to in this place. The reader's 

 attention is here drawn to the succession of doctrines held on the 

 subject of heat, in order to prevent him from falling into the some- 

 what prevalent mistake of considering that atoms are proved to have 

 a real existence. They are mental creations, like phlogiston or caloric, 

 and hold their place in our conception for the same reason which on-ce 

 made of these last important scientific doctrines ; that is, they enable 

 us to form consistent representations in our minds of a certain range 



