Recently I wrote for GESIS on the history of science communication and popularized science. It’s an interesting question when the practice started. As with so many phenomena, this is mostly down to the historian’s favorite thing: a definition.
In terms of Germany, and in terms of high visibility popularized science, Justus von Liebig’s Familiar Letters on Chemistry are a good starting point. Read more about what I had to say about these, and about science communication in the twentieth and twenty-first centuries over at the GESIS Blog.