We have, I think, found that throughout the progress of chemical science two dangers have beset the student. He has been often tempted to accumulate facts, to amass analytical details, to forget that he is a chemist in his desire to perfect the instrument of analysis by the use of which he raises the scaffolding of his science; on the other hand, he has been sometimes allured from the path of experiment by his own day-dreams. The discoveries of science have been so wonderful, and the conceptions of some of those who have successfully prosecuted science have been so grand, that the student has not unfrequently been tempted to rest in the prevailing theories of the day, and, forgetting that these ought only "to afford peaceful lodgings to the intellect for the time," he has rather allowed them to circumscribe it, until at last the mind "finds difficulty in breaking down the walls of what has become its prison, instead of its home." We may think that Dumas fell perhaps slightly into the former of these errors, when he did not allow his imagination a little more scope in dealing with the conception of "atom" and "molecule," the difference between which he had apprehended but not sufficiently marked by the year 1826 (see p. 261). We know, from his own testimony, that Liebig once fell into the latter error and that the consequences were disastrous. "I know a chemist"--meaning himself--"who ... undertook an investigation of the liquor from the salt-works. He found iodine in it, and observed, moreover, that the iodide of starch turned a fiery yellow by standing over-night. The phenomenon struck him; he saturated a large quantity of the liquor with chlorine, and obtained from this, by distillation, a considerable quantity of a liquid which coloured starch yellow, and externally resembled chloride of iodine, but differed from this compound in many properties. He explained, however, every discrepancy with satisfaction to himself; he contrived for himself a theory. Several months later, he received a paper of M. Balard's," announcing the discovery of bromine, "and on that same day he was able to publish the results of experiments on the behaviour of bromine with iron, platinum, and carbon; for Balard's bromine stood in his laboratory, labelled _liquid chloride of iodine_. Since that time he makes no more theories unless they are supported and confirmed by trustworthy experiments; and I can positively assert that he has not fared badly by so doing."