Materialism is a form of monism that holds that everything can be reduced to a material explanation. In materialist explanations of reality, there is no room for explanations relying on truly supernatural phenomena.
Materialists and Spiritualists?
It is in this sense that it makes sense to distinguish between 'materialist' and 'spiritualist' explanations of Mesmerism or Spiritualism. Materialists would argue that the phenomena have a material cause, such as a previously undiscovered or invisible force (animal magnetism, electricity etc). On the other hand, 'spiritualists' (with a small 's'), would argue these phenomena reveal the existence of other forces beyond the material world.
Although this distinction may look messy, it is arguably very important. Previously undiscovered but nonetheless natural forces could be studied by nineteenth-century science, whereas supernatural forces would have lain outside the remit of the physical sciences, in the fields of religion or philosophy that were slowly being distinguished from the natural sciences in this period.
The claims of the materialists were claims to be heard by scientists, whereas spiritual explanations placed these phenomena outside the fluid boundaries of the newly defined realm of science.