The starting point for this study was the resistant nature of prior knowledge in conceptual change from natural numbers to rational numbers observed in our previous study. Thus, in this study the effects of deliberately teaching the abstraction of the density of numbers on the number line was tested in a quasi-experimental study at the beginning of students' first course in calculus. The results suggest a significant and stable effect in the post test between the test group and control group. This effect had an important relation to the level and quality of abstraction of the number line, which was found in the short essays written by the participants of the test group during the intervention. (Contains 1 footnote and 1 figure.) [For complete proceedings, see ED500858.]