Materialism and Monism:
Oxford Dictionary Definition:
Materialism: 'The theory or belief that nothing exists except matter and its movements and modifications.'
Monism: 'A theory or doctrine that denies the existence of a distinction or duality in a particular sphere, such as that between matter and mind, or God and the world.'
[1] Materialism was the world belief that opposed dualism. The core beliefs of materialism were that the universe was made up of one material. It proposed that the world did not have faceted layers beyond our understanding. An example of a materialistic perspective can be seen in some aspects of mesmerism. Menser's original thesis on Animal Magnetism postulated that there was a physical fluid passing between animals that could be harnessed. [2: Kaplan, F. ‘“The Mesmeric Mania”: The Early Victorians and Animal Magnetism’ Journal of the History of Ideas, 35 (1974), 692] The emphasis here should be on the physicality of the force Mesmer refers to. Although it was thought of as an invisible fluid, Mesmer suggested that is was a physical, natural force and not one of a supernatural nature (Other beliefs around Animal Magnetism did exist that promoted a spiritualist reading of the phenomena).
For a person to hold materialist views in one area of science did not mean that these people were totally materialist in their outlook. Andrew White's work from 1896 shows this clearly. White dismissed many of the miracles that were reported in the medieval era as mis-communicated, or simply false reports, talking about the fetishism that surrounded relics [3: White, Andrew Dickson, ‘From Miracles to Medicine’, in A History of the Warfare of Science With Theology, 2 (1896), 1-66.]. Whilst this takes a more materialistic approach to the miracles reported, claiming they were not supernatural, White did not dismiss the existence of miracles outright. He took a materialist approach in the sense that he viewed many were not supernatural, but was not a materialist in his world view.
Materialism, and possibly pure monism, was present in beliefs leading in to our period in question. Positivism roots itself in the physical. Although there was 'a revolt against positivism' emerging [4: Michael Saler, ‘Modernity and Enchantment: A Historiographic Review’, American Historical Review, 111:3 (2006), 697.], the view of superstitious cultures as inferior because of their superstition shows that there was a degree of materialism ingrained into western society. [5: Michael Saler, ‘Modernity and Enchantment: A Historiographic Review’, American Historical Review, 111:3 (2006), 696.] Max Weber suggests that this notion of beleif in the 'disenchantment' of the time was used, on some level, as an elitest tool to promote hierachy. [6: Michael Saler, ‘Modernity and Enchantment: A Historiographic Review’, American Historical Review, 111:3 (2006), 692-716.]
Monism is a broader theory of singularity within philosophy. It refers to a belief in a singular 'thing', that 'thing' changing depending on what the monism is applied to. Materialism, at it's purist form or as a world view, is a form of monism.