"Well, Van Helmont was in just your position. He could not know anything about the cause of this diminution, because he was ignorant of the composition of the air, which was not discovered until the next century by the celebrated French chemist Lavoisier. Now, this is how Lavoisier arrived at this important discovery. In the first place, he knew that metals, when they are calcined,—that is to say, when they are exposed to the action of fire,—increase in weight. This fact had been remarked before his time by Dr. Jehan Rey, under the following circumstances: A druggist named Brun came one day to consult the doctor. Rey asked to be allowed to feel his pulse.
"'But I am not sick,' cried the druggist.
"'Then what are you doing here?' said the doctor.
"'I come to consult you.'
"'Then you must be sick.'
"'Not at all. I come to consult you not for sickness, but in regard to an extraordinary thing which occurred in my laboratory.'
"'What was it?' asked Rey, beginning to be interested.
"'I had to calcine two pounds six ounces of tin. I weighed it carefully and then calcined it, and after the operation I weighed it again by chance, and what was my astonishment to find two pounds and thirteen ounces! Whence come these extra seven ounces? That is what I could not explain to myself, and that is why I came to consult you.'
"Rey tried the same experiment again and again, and finally concluded that the increase of weight came from combination with some part of the air.
"It is probable that this explanation did not satisfy the druggist; and yet the doctor was right. The increase came from the combination of the metal with that part of the air which Lavoisier called oxygen. That great chemist, after long study, declared that air was not a simple body, but that it was a composite formed of two bodies, of two gases,—oxygen and nitrogen. This opinion, running counter as it did to all preconceived ideas, raised a storm around the head of the learned man. He was looked upon as a fool, as an imbecile, as an ignoramus. That is the usual way.