Why did some chemists resist using physical methods such as vapour-density measurements?

Many chemists in the mid-19th century were reluctant to use physical methods, such as vapour-density measurements, to solve chemical problems because they viewed these techniques as abstract and potentially unreliable for determining atomic weights and molecular formulas.1

At the time, the chemical community was divided by a chaotic multiplicity of competing, incompatible atomistic systems. Because there was no direct way to weigh individual atoms or molecules, many chemists preferred to rely on purely chemical evidence rather than physical data. Furthermore, the prevailing belief that chemical forces were essentially electrical in nature created a theoretical barrier; for example, it was thought that two hydrogen atoms would repel each other due to identical electrical charges, making it impossible for them to join into the polyatomic molecules suggested by Avogadro’s hypothesis.12

It was not until Stanislao Cannizzaro demonstrated that a consistent system based on Avogadro’s ideas could reconcile both physical and chemical evidence that these methods gained widespread acceptance.1