In Coleridge's Biographia Literaria, about fancy and imagination he writes "the difference between the two is the same as the difference between a mechanical mixture and a chemical compound."
Coleridge, in this famous work of criticism, argues that fancy and imagination are actually two separate entities, rather than being either synonymous or words used to describe differing intensities of the same object. The above quote therefore supports this view by arguing that the two terms are, in their very essence, essentially different one from the other and cannot be compared. Note how Coleridge elaborates on this difference between fancy and imagination:
Repeated meditations led me first to suspect... that fancy and imagination were two distinct and widely different faculties, instead of being, according to the general belief, either two names with one meaning, or at furthest the lower and higher degree of one and the same power.
To Coleridge, therefore, one of his central arguments in this work of criticism is that the two terms "fancy" and "imagination" are separate entitites and that they are not synonymous in any way. He goes on to develop his own definition of both of these terms later on in this text, but the quote given above seeks to provide a practical analogy of how these two terms are different in their very nature and therefore cannot be confused one for the other.