In the spring of 1781, Frenchman Antoine Lavoisier's wife, Marie, trans lated a paper by English scientist Robert Boyle into French. The paper described an experiment with tin during which Boyle had noted an unexplained weight change when the tin was heated.
Boyle, like most scientists, was content to assume that the extra weight had been “created” during his chemical experiment.
Lavoisier scoffed at the notion of mysterious creation or loss of mass (weight) during reactions. He was convinced that chemists' traditional experimental approach was inadequate. During experiments chemists carefully observed and described changes in a substance. Lavoisier claimed it was far more impor tant to record what could be measured.
Weight was one property he could always measure.
Lavoisier decided to repeat Boyle's experiment, carefully measure weight, and discover the source of the added weight. Antoine carried a small sheet of tin to his delicate balancing scales and recorded its weight. Next he placed the tin in a heat resistant glass flask and sealed its lid to contain the entire reaction within the flask.
He weighed the flask (and the tin inside) before heating it over a burner. A thick layer of calx (a light gray tarnish) formed on the tin as it heated as Boyle had described in his paper.
Lavoisier turned off the burner, let the flask cool, and then reweighed it. The flask had not changed weight. He pried off the flask's lid. Air rushed in, as if into a partial vacuum.
Antoine removed and weighed the calx covered tin. It had gained two grams of weight (as had Boyle's).
Lavoisier deduced that the weight had to have come from the air inside the flask and that was why new air rushed into the flask when he opened it. The tin gained two grams as it mixed with air to form calx. When he opened the lid, two grams of new air rushed in to re-
place the air that had been absorbed into calx.
He repeated the experiment with a larger piece of tin. However, still only two grams of air were absorbed into calx. He ran the experiment again and measured the volume of air that was absorbed into calx, 20 per cent of the total air inside the flask.
He concluded that only 20 percent of air was capable of bonding with tin. He realized that this 20 percent of air must be the “pure air” Priestley had dis covered in 1774, and Lavoisier named it “oxygen.”
Through further experiments Lavoisier realized that he had proved something far more important. Boyle thought weight, or matter, was “created” during experiments. Lavoisier had proved that matter was nei ther created nor lost during a chemical reaction. It always came from someplace and went to someplace. Scientists could always find it if they measured carefully.
The all important concept of conservation of matter had been discovered. However, Lavoisier didn't release this principle until he published his famed chemistry textbook in 1789.
