Attempts are made to define the degrees of stiffness of batters and doughs, but these distinctions are not very accurate. A "pour batter" is liquid enough to pour, and a "dough batter" soft enough to drop from a spoon; a "soft dough" is next in grade, and "dough" is the stiffest of all.
To understand proportioning the ingredients, the nature of the ingredients when heated must be taken into account. Butter and other fats melt when heated, and behave like a liquid in the mixture. Therefore, when there is a very large amount of butter, no other wetting is necessary, as in pound cake. We may make a scale, with a thin popover mixture at one extreme, with no butter in it, and the stiff pound cake at the other, with butter the only liquid (except the flavoring). Between these two are the mixtures of medium stiffness, with both butter and liquid. This general rule may be given: As the quantity of butter is increased, the batter must increase in stiffness, and there must be either less liquid or more flour.
A beaten egg looks like a liquid and behaves so during the mixing, but in the oven it stiffens. For this reason we can make a sponge cake with many eggs and no liquid in the mixing, and use no other leavening agent than the air beaten into the egg.
One old-fashioned rule for sponge cake reads: Take the weight of the eggs in sugar and half their weight in flour, with the juice and rind of a lemon for ten eggs. Such a rule was adapted to the days when eggs were cheap. We should now use fewer eggs in sponge cake, and this means that water and baking powder must replace the eggs omitted.