meet in a point, they form one solid angle, which comes next in order to the most obtuse of the plane angles. And when four such angles are produced, the first solid figurei.e., the tetrahedron or pyramid (molecule of fire). is constructed, which divides the whole of the circumscribed sphere into equal and similar parts. And the second solidi.e., the octahedron (molecule of air). is formed from the same triangles, but constructed out of eight equilateral triangles, which produce one solid angle out of four planes; and when six such solid angles have been formed, the second body in turn is completed.