 Material unity. A more recent approach to the connection between different research areas has focused on a material level of scientific practice, with attention to the use of instruments and other material objects (Galison 1997, Bowker and Star 1999). For instance, the material unity of natural philosophy in the 16th and 17th centuries relied on the circulation, transformation and application of objects, in their concrete and abstract representations (Bertoloni-Meli 2006). The latter correspond to the imaginary systems and their representations, which we call models. The evolution of objects and images across different theories and experiments and their developments in 19th-century natural philosophy provide a historical model of scientific development; but the approach is not meant to illustrate reductive materialism, since the same objects and models work and are perceived as vehicles for abstract ideas, institutions, cultures, etc., or prompted by them (Cat 2013). On one view, objects are regarded as elements in so-called trading zones (see below) with shifting meanings in the evolution of 20th-century physics, such as with the cloud chamber which was first relevant to meteorology and next to particle physics (Galison 1997). Alternatively, material objects have been given the status of boundary objects, which provide the opportunity for experts from different fields to collaborate through their respective understanding of the system in question and their respective goals (Bowker and Star 1999).