Therefore, we also carry out experiments in which reaction conditions are held constant, allowing the changes in reactivity due to structural or compositional evolution to be observed directly. Fig. shows the result of an experiment probing the time dependence of the 1500 K O 2 reactivity of a typical Hf NP, and for comparison, the results of analogous experiments for 1500 K O 2 reaction with similar size graphite, carbon black, and silicon NPs. The comparison to systems where the chemistry is better understood is important in interpreting the Hf NP behavior. Fig. shows the absolute mass vs. time data, where the slopes of M vs. time give the absolute rates of mass loss, with different mass scales for each NP reflecting differences in atomic masses and NP sizes.