Hi! You're right that the word "thick" can have a surprising range of meanings and uses, and "fall thick" adds an interesting layer to that.
In the examples you gave—"snow/flakes/rain/sparks/tears fell thick"—this phrase is a poetic and somewhat figurative way to describe something falling heavily, abundantly, or densely. It conveys the idea that the objects are falling in such a way that they seem to fill the air or the space between them is minimal.
To break it down:
1. Common usage: In phrases like "rain is falling thick and fast," "fall thick" is used in a descriptive and almost visual way to describe how intensely something is falling. This is more common in literary or poetic language but understandable in everyday speech to mean a lot of something is falling at once.
2. Longfellow's usage: The line you mentioned from Longfellow—"But the hopes of youth fall thick in the blast"—is definitely a figurative use of the phrase. Here, "fall thick" is describing how the hopes of youth are being lost or dashed in large numbers, much like snow or rain might fall in a storm. The metaphor conveys the fragility of youthful hopes being blown away by life's hardships, much as snowflakes would be scattered in the wind.
In this case, Longfellow uses "fall thick" not just to describe a physical event, but to evoke a sense of overwhelming loss, where many hopes are being destroyed all at once, much like in a storm where things are blown away in abundance.
So, to summarize:
In common usage, "fall thick" can describe something falling heavily or in large amounts (e.g., rain, snow, etc.).
In figurative usage, like Longfellow's poem, it describes the heavy or abundant loss of something intangible, like hopes or dreams.
Both are valid uses, but in poetry, it's often used more metaphorically for emotional or conceptual ideas.