388 THE HISTORY OF MEDIEVAL EUROPE reference to the diseases and treatment of their patients, and many rulers kept astrologers at their courts. Even bishops and popes were at times known to consult them. The alchemists tried to convert other metals into gold, and often proceeded toward this goal by mystic methods with incantations and useless ceremonial. However, the alchem- ists of the thirteenth century were more sober and scientific and less superstitious than those of the Greek-speaking or Arabian worlds. They were already on the road to modern chemistry, and there is to-day a tendency to return to their belief that one primal matter lies behind the chemical ele- ments. Peter of Abano, however, who was probably the most learned man living around the year 1300, despaired of any such discovery as that of atomic weights, declaring it impossible to find the quantities and weights of the con- stituent elements in any object. Such uncertainty concern- ing the composition of bodies was one reason for the belief in occult virtues. Scientific apparatus was still in a primitive state and the experimental method and mathematical accuracy of mod- Experiment ern sc * ence were not Y et m existence. But men and like the alchemists and architects experimented a good deal in their own way and attained to some important discoveries as a result. The mariner's com- pass with its magnetic needle, gunpowder,, and magnifying lenses for eyeglasses all first became known in the twelfth and thirteenth centuries. We have already mentioned chimney flues, lead plumbing, and glass windows. New dyes and industrial processes were discovered, and mechanical clocks were a medieval invention. Clocks and lenses were later to prove of great help in scientific investigation, since the one enables time to be measured accurately, while the other, when developed into telescope and microscope, en- ables one to study much that is invisible to the naked eye. Many learned men, from Adelard of Bath in the early twelfth to Roger Bacon in the thirteenth century, ad- vocated experimentation as well as reading authorities as a method of discovering truth. Roger Bacon, in a work
