About the middle of the year 1817, his grief, slowly abating, left him a prey to solitude and defenceless under the monotony of the life he was leading, which heavily oppressed him. At first he struggled bravely against the allurements of Science as they gradually beset him; he forbade himself even to think of Chemistry. Then he did think of it. Still, he would not actively take it up, and only gave his mind to his researches theoretically. Such constant study, however, swelled his passion which soon became exacting. He asked himself whether he was really bound not to continue his researches, and remembered that his wife had refused his oath. Though he had pledged his word to himself that he would never pursue the solution of the great Problem, might he not change that determination at a moment when he foresaw success? He was now fifty-nine years old. At that age a predominant idea contracts a certain peevish fixedness which is the first stage of monomania. Circumstances conspired against his tottering loyalty. The peace which Europe now enjoyed encouraged the circulation of discoveries and scientific ideas acquired during the war by the learned of various countries, who for nearly twenty years had been unable to hold communication. Science was making great strides. Claes found that the progress of chemistry had been directed, unknown to chemists themselves, towards the object of his researches. Learned men devoted to the higher sciences thought, as he did, that light, heat, electricity, galvanism, magnetism were all different effects of the same cause, and that the difference existing between substances hitherto considered simple must be produced by varying proportions of an unknown principle. The fear that some other chemist might effect the reduction of metals and discover the constituent principle of electricity,--two achievements which would lead to the solution of the chemical Absolute,--increased what the people of Douai called a mania, and drove his desires to a paroxysm conceivable to those who devote themselves to the sciences, or who have ever known the tyranny of ideas.