It's an illusion, though, isn't it?
"1. trans. To thicken, condense.
[...] fig. 1732 Historia Litteraria III. 249 When the Subject is limpid of it self, he frequently inspissates it, by throwing in a heap of Circumstances not Essential to it. 1884 E. E. HALE Xmas in Narrag. v. 117 No method..by which you can inspissate entertainingness into a dull article.
2. intr. To become thick or dense, to thicken.
[...] 1836 MACGILLIVRAY tr. Humboldt's Trav. xvii. 225 Until the yolk..has time to inspissate."