The two intended to write a two volume work and Treatise on Natural Philosophy (1867) was to be the first of the volumes. However the second volume was never written.
Ang dalawang inilaan upang makapagsulat ng isang dalawang dami ng trabaho at Treatise sa Natural Philosophy (1867) ay ang unang ng volume. Subalit ang ikalawang volume ay hindi nakasulat.