All sorts of scientific ideas have been amended or abandoned in the past few centuries. In fact such a huge variety of scientific beliefs have changed since the 18th and 19th centuries that its really not possible to give an answer that would cover them in any detail.

Could you be more specific about the kinds of things you are interested in? I could try to answer them myself if I know how, or perhaps I could suggest some history of science books that would interest you.

The theory of the four temperaments, originally proposed by Hippocrates was almost universally believed to be true throughout the 18th century and a good deal into the 19th century.

This again led to the belief that bloodletting was an effective remedy for almost all kinds of illnesses, as they thought they were caused by the presence of an excessive amount of one or more of the four types of bodily fluid connected to the temperaments.