PROF. MCLENNAN'S lelter in NATURE of May 14 (p. 29) makes it desirable that I should record that my experiments continue to be in contradiction to those he describes. Whether the activity of potassium is due to an extraneous impurity or to the element itself, it is to be expected that some separation of the activity should be possible. For the last year and a half I have been trying to effect such a separation without result. All samples of the same pure salt which I have procured or prepared are identical in activity. In one experiment a sample of the sulphate was crystallised twenty-two times, in another a sample of the nitrate was crystallised eighteen times without introducing any difference between the final products; in a third a sample of potassium sulphate was prepared direct from wood ashes, and found to agree with a commercial sample prepared from the Stassfurt deposits. I propose now to prepare a third sample direct from felspar.