Mass density is a 3-form, just like electric charge density. Hence it's exterior derivative is 0. This implies that there is a 2-form whose exterior derivative is the density 3-form. If this density is the electric charge density, then the name of this 2-form is "excitation" (D). But what is the name of it in the case when we speak about mass density instead off electric charge density? And where is it used?