Mass of a gas sample
The concept of a mole(c. 1805), and Avogadro's number(1811) occurred after Dalton formulated his law (1801). So he would not have set out to measure a "mole of gas" as the modern formulation sets out. This history of Dalton may be useful to you: Google Book Search Most of his work centered around pressure readings, as his early interests were meterological.