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20 have passed some years of his life in Egypt, and to have been instructed in science by the priests of Thebes and Memphis; and it is therefore possible that he may have been influenced by the Egyptian teaching in the formulation of his cosmological theories.

It is significant of the tenacity with which the mind clings to dogma and reveres authority that the teaching of Thales should have survived through the space of twenty-four centuries. It can be shown to have affected the course of chemical inquiry down to the close of the eighteenth century. It influenced the experimental labours of philosophers so diverse in character as Van Helmont, Boyle, Boerhaave, Priestley, and Lavoisier—all of whom made attempts to prove or disprove its adequacy. Van Helmont, indeed, was one of the most strenuous supporters of the doctrine of Thales, and sought to establish it by observations which, in the absence of all knowledge of the true nature of air and water, seemed at the time irrefutable. Perhaps the one most frequently cited is his observation on the growth of a plant which apparently had no other form of sustenance than water. He describes how he planted a willow weighing 5 lbs. in 200 lbs. of earth previously dried in an oven.

The plant was regularly watered, when at the end of five years it was found to weigh 169 lbs. 3 oz., whereas the earth, after redrying, had lost