 Spinoza engaged in very little experimentation of his own, but he did show some interest in the experimental results of others. His letters contain several discussions of experimental and theoretical dioptrics, to be expected from a lens grinder and man of letters, as well as discussions of recent observations of comets and whether they can be explained on Cartesian principles, of the new microscope, and of medical and alchemical experiments. Far and away the most famous and significant discussion of experimentation are found in his exchanges with Henry Oldenburg, the first Secretary of the Royal Society, who operated largely as the mouthpiece of Robert Boyle, and in particular those concerning Boyle's experiments on solidity, fluidity, and on nitre. The latter of these is the most telling of Spinoza's attitude towards the relevance of experiment to theorizing about nature. Boyle had claimed to show that nitre (potassium nitrate) is a chemical compound rather than a mixture, by having decomposed it into fixed and volatile parts (potash and spirit of nitre), and then recombining them into nitre with little or no loss of quantity. The disparate properties of the components, he argued, showed that the nitre itself was a compound, in which the components were altered and transformed, rather than a mere mixture. This, in turn, suggested that the basic constituents of the components were preserved through the interaction, confirming the corpuscular chemical theory over the scholastic view that chemical transformation involves the substantial form of given matter being destroyed and replaced by some other substantial form.