5 Typed letters between Richard Millikan and Charles F., Sr., discussing potential flaws in Brush's free-fall experiments which Brush believed demonst Show more5 Typed letters between Richard Millikan and Charles F., Sr., discussing potential flaws in Brush's free-fall experiments which Brush believed demonstrated a different mass-weight ratio in various elements, and Brush's later experiments with the Bureau of Standards exploring spontaneous generation of heat in igneous rocks. Letters dated 1923, 1924, 1926. Charles F. Brush, Sr., Papers Series 1: Correspondence Subseries 3: Business and Professional (1876-1931) Box 5 Folder 6 Show less