974 



SCIENCE 



[N. S. Vol. XXVII. No. 704 



know what he was talking about, but his 

 words made an impression. Here was a 

 man who, according to the statement 

 quoted, was actually doing things of value 

 to the world. Whether I learned anything 

 from his lectures or not was a secondary 

 matter. It was worth much to be per- 

 mitted to see him and to hear him talk. 



Dr. Doremus did not teach chemistry at 

 the college in my day, but shortly after- 

 ward he gave some popular lectures on 

 chemistry at the Cooper Institute, in the 

 course of which he performed extremely 

 striking experiments, many of which I can 

 remember as clearly as if they had been 

 performed yesterday. In fact, I have 

 never seen more brilliant chemical lecture 

 experiments. The hall was crowded and 

 I am sure the lectures set many to think- 

 ing. I have always felt that my own in- 

 terest in chemistry, which soon became ab- 

 sorbing, was due to what I saw and heard 

 in these lectures. 



With this brief reference to chemistry at 

 the college nearly fifty years ago let me 

 pass to chemistry as it was in the world 

 at large at that time. We often hear the 

 statement that chemistry has been com- 

 pletely revolutionized within a compara- 

 tively brief period. I have been hearing 

 that statement ever since I have known 

 anything about chemistry. After all? 

 progress in chemistry has not been by 

 revolution, but by evolution. Probably the 

 nearest to a revolution was that which hap- 

 pened during the last quarter of the eight- 

 eenth century -when Priestley and Scheele 

 and Lavoisier explained the nature of com- 

 bustion and paved the way to the over- 

 throw of the theory of phlogiston which 

 had so long controlled the views of chem- 

 ists. But that theory was not overthrown 

 in a day or in a year. Priestley and 

 Scheele, whose discovery of oxygen led to 

 Lavoisier's work on combustion, both re- 



mained phlogisticians to the end of their 

 days, as did most of their contemporaries. 



Within the last half century the change 

 that has made the most impression on the 

 outside world and has led to the common 

 belief that the older views have been com- 

 pletely given up and that radically new 

 ones have taken their place, is that which 

 is due to the gradual acceptance of what 

 is generally known as the law of Avogadro. 

 The conception embodied in this law is very 

 simple. It is that the number of molecules 

 contained in a given volume of a gas or 

 vapor is the same, no matter what the gas 

 or vapor may be, provided only that the 

 temperature and pressure are the same. 

 That it is difficult to prove the truth of 

 this statement is evident from the fact that 

 it was nearly a half century after it was 

 propounded by Avogadro before it came 

 to be generally accepted. Few, if any, 

 accepted it at the time it was first put for- 

 ward. The leaders tried to apply it to 

 well-known facts and gave it up. And yet, 

 in the light of facts discovered later, it 

 came to be recognized as a fundamental 

 truth of great value. 



When Gibbs was teaching chemistry in 

 the old Free Academy, Avogadro 's law was 

 not taught in this country and only a few 

 of the younger teachers were beginning 

 to teach it and to use it in Italy and 

 Germany. It was a most confusing time 

 for the student. According to the pre- 

 vailing system, to take an example, the 

 atomic weight or the combining weight or 

 the equivalent of oxygen was 8, whereas, 

 according to Avogadro, it was 16. And 

 yet it was the same old oxygen that had 

 been discovered by Priestley and Scheele, 

 and it supported combustion in exactly the 

 same way whether we assigned to it the 

 atomic weight 8 or 16. How could both 

 be true? I remember in 1867, when I 

 finally decided to give up medicine and 

 study chemistry, meeting a man who knew 



