Dalton concluded: "the ultimate particles of all homogeneous bodies are perfectly alike in weight, figure, etc... Those ultimate particles are known as atoms."    He also concluded that if two elements combine to form more than one compound, the amount of one element which combines with a fixed amount of the other element are in ratios of small whole numbers.  This was called the law of multiple proportions.  At this point the atom was still believed to be indivisible. 

The English physicist, J.J. Thomson would devise a model of the atom which was no longer an indivisible sphere, but rather a combination of a positively charged "pudding" and negatively charged electrons "plums" similar to the seeds suspended in the "meat" of a watermelon. When voltage was applied to Thomson's apparatus, light was seen leaving the cathode which were called cathode rays.  They caused a spot of light on a zinc sulfide screen.  Adjusting the electric field or the magnetic field would cause the "ray" to be displaced.  From this experiment and based on other observations, Thomson concluded that:

Thomson's apparatus built for observing this effect can be seen in figure 1.1.3.