We claim the ornamental design for a device for diffusing substances, as shown and described.
FIG. 1 is a top, right perspective view of a device for diffusing substances showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view thereof.
The broken lines in the drawings are for illustrative purposes only and form no part of the claimed design. The dash-dot broken lines in the drawings delineate the boundaries of the claimed subject matter, wherein the claimed subject matter includes shading and the unclaimed subject matter does not include shading.