Does "parts" mean molar or mass?

Researching old (like century-old) papers it is common for chemists to describe preparations in terms of parts – e.g., "1 part water, 2 parts sulfuric acid."


Is there a reliable convention on whether this means molar parts or mass parts? I.e., in the above example is the formula 1:2 parts *by mass* or *by molecules*?


