Despite time constraints imposed on him by the expansion of his business, Boulton continued his "philosophical" work (as scientific experimentation was then called). He wrote in his notebooks observations on the freezing and boiling point of mercury, on people's pulse rates at different ages, on the movements of the planets, and on how to make sealing wax and disappearing ink. However, Erasmus Darwin, another fellow enthusiast who became a member of the Lunar Society, wrote to him in 1763, "As you are now become a sober plodding Man of Business, I scarcely dare trouble you to do me a favour in the ... philosophical way."