The continuity equation. A local conservation law.
Sander Bais. Gijs Mathijs Ontwerpers, Amsterdam
Sander Bais: De natuurwetten. Iconen van onze kennis, Salomé - Amsterdam University Press, 2005. <a href="//commons.wikimedia.org/wiki/Special:BookSources/90_5356_714_3" title="Special:BookSources/90 5356 714 3">ISBN 90 5356 714 3</a>. NUR 616/754. Page 32.