This is the 4408th most frequent Russian word.
веществами
"Substances" (instrumental plural form of "вещество").
In this sentence, 'веществами' refers to physical or chemical materials or substances contained within a briefcase, indicating plurality and instrumental case.
Портфель был наполнен какими-то странными веществами.
The briefcase was filled with some strange substances.
Here, 'веществами' highlights the objects of scientific study or experimentation, functioning as the instrumental case to denote the usage or manipulation of the substances.
Ученые проводят эксперименты с новыми веществами.
Scientists are conducting experiments with new substances.
In this context, 'веществами' implies the types of substances being referenced, particularly toxic ones, and utilizes the instrumental case showing the focus of their interest.