Another point which we notice in the life-work of these two chemists is their untiring labour. They were always at work; wherever they might be, they were ready to notice passing events or natural phenomena, and to draw suggestions from these. As Davy proved the elementary character of iodine and established many of the properties of this substance during a visit to Paris, so we find Dumas making many discoveries during brief visits paid to his friends' laboratories when on excursions away from Paris. During a visit to Aix-les-Bains, he noticed that the walls of the bath-room were covered with small crystals of sulphate of lime. The waters of the bath, he knew, were charged with sulphuretted hydrogen, but they contained no sulphuric acid, nor could that acid be detected in the air of the bath-rooms. This observation was followed up by experiments which proved that a porous material, such as a curtain or an ordinary plastered wall, is able to bring about the union of oxygen with sulphuretted hydrogen, provided moisture be present and a somewhat high temperature be maintained. Again, we find Liebig and Dumas characterized by great mental honesty. "There is no harm in a man committing mistakes," said Liebig, "but great harm indeed in his committing none, for he is sure not to have worked.... An error you have become cognizant of, do not keep in your house from night till morning." Students of science, more than any other men, ought to be ready to acknowledge and correct the errors into which they fall. It is not difficult for them to do this: they have only to be continually going to Nature; for there they have a court of appeal always ready to hear their case, and to give an absolutely unbiased judgment: they have but to bring their theories and guesses to this judge to have them appraised at their true value. FOOTNOTES: [13] "In reviewing once more the facts elicited by our inquiry, we find them arranged around a common centre, a group of atoms preserving intact its nature, amid the most varied associations with other elements. This stability, this analogy, pervading all the phenomena, has induced us to consider this group as a sort of compound element, and to designate it by the special name of _benzoyl_."--Liebig and Woehler, 1832. [14] "Animal Chemistry, or Chemistry in its Applications to Physiology and Pathology," 1842. "Researches on the Chemistry of Food," 1847. "The Natural Laws of Husbandry," 1862. CHAPTER VII. MODERN CHEMISTRY. On p. 162 I referred to the work of the German chemist Richter, by which the _equivalents_ of certain acids and bases were established. Those quantities of various acids which severally neutralized one and the same quantity of a given base, or those quantities of various bases which severally neutralized one and the same quantity of a given acid, were said to be equivalent. These were the quantities capable of performing a certain definite action.