The experiment of Zola is as if a professor of chemistry, knowing something of the properties of given substances desiring to see how a certain molecule would behave itself in the presence of a certain other molecule, hitherto untried in this connection, instead of going into his laboratory and bringing the molecules together and observing what they actually did, should quietly sit before his desk and write off a comfortable account of how he thought these molecules would behave, judging from his previous knowledge of their properties. It is still more interesting to find that Zola is apparently unconscious of the difference between these two modes of experiment. About this unconsciousness I have my own theory. I think it entirely probable that if these two kinds of experiment were described to Zola he would maintain with perfect good faith that they were exactly the same. There is a phase of error--perhaps we may call it hallucination--in which certain sorts of minds come to believe that two things which have been habitually associated are always the same. For instance, a friend of mine has told me that a certain estimable teacher of the French language, who, after carrying on his vocation for many years, during which English and French became equally instinctive tongues to him, was accustomed to maintain that English and French were absolutely one and the same language. "When you say _water_," he was accustomed to argue to my friend, "you mean water; when I say _l'eau_ I mean water; _water--l'eau_, _l'eau--water_; do you not see? We mean the same thing; it is the same language." However this may be, nothing is clearer than that Zola's conception of an experiment is what I have described it--namely, an evolving from the inner consciousness of what the author _thinks_ the experimental subjects would do under given circumstances. Here are some of Zola's own words: and surely nothing more naïve was ever uttered: "The writer" (of the novel) "employs both observation and experiment. The observer gives the facts as he has observed them ... and establishes the solid ground on which his characters shall march, and the phenomena shall develop themselves. _Then the experimenter appears and conducts the experiment; that is to say_" (I am quoting from M. Zola) "_he moves the characters in a particular story to show that the sequence of facts will be such as is determined in the study of phenomena_." That is to say, to carry Zola's "experiment" into chemistry: knowing something of chlorine and something of hydrogen separately, a chemist who wishes to know their behavior under each other's influence may "experiment" upon that behavior by giving his opinion as to what chlorine and hydrogen would likely do under given circumstances.