Trial and error--alternate guess and rejection, generation and destruction of sentiments and beliefs--is among the most widespread facts of human intelligence.[34] Even those ordinary minds, which in mature life harden with the most exemplary fidelity into the locally prevalent type of orthodoxy,--have all in their earlier years gone through that semi-fluid and indeterminate period, in which the type to come is yet a matter of doubt--in which the head might have been permanently lengthened or permanently flattened, according to the direction in which pressure was applied. [Footnote 33: It is not often that historians of science take much pains to preserve and bring together the mistaken guesses and tentatives which have preceded great physical discoveries. One instance in which this has been ably and carefully done is in the 'Biography of Cavendish,' the chemist and natural philosopher, by Dr. Geo. Wilson. The great chemical discovery of the composition of water, accomplished during the last quarter of the eighteenth century, has been claimed as the privilege of three eminent scientific men--Cavendish, Watt, and Lavoisier. The controversy on the subject, voluminous and bitter, has been the means of recording each successive scientific phase and point of view. It will be found admirably expounded in this biography. Wilson sets forth the misconceptions, confusion of ideas, approximations to truth seen but not followed out, &c., which prevailed upon the scientific men of that day, especially under the misleading influence of the "phlogiston theory," then universally received. To Plato such a period of mental confusion would have been in itself an interesting object for contemplation and description. He might have dramatised it under the names of various disputants, with the cross-examining Elenchus, personified in Sokrates, introduced to stir up the debate, either by first advocating, then refuting, a string of successive guesses and dreams (Charmidês, 173 A) of his own, or by exposing similar suggestions emanating from others; especially in regard to the definition of _phlogiston_, an entity which then overspread and darkened all chemical speculation, but which every theorist thought himself obliged to define. The dialogues would have ended (as the Protagoras, Lysis, Charmidês, &c., now end) by Sokrates deriding the ill success which had attended them in the search for an explanation, and by his pointing out that while all the theorists talked familiarly about _phlogiston_ as a powerful agent, none of them could agree what it was. See Dr. Wilson's 'Biography of Cavendish,' pp. 36-198-320-325, and elsewhere.] [Footnote 34: It is strikingly described by Plato in one of the most remarkable passages of the speech of Diotima in the Symposion, pp. 207-208.]