He returned to Paris, where fresh sorrow awaited him: Camille, his youngest child, only two years old, was seriously ill. He watched over her night after night, spending his days at his task in the laboratory, and returning in the evening to the bedside of his dying child. During that same period he was asked for an article on Lavoisier by J. B. Dumas, who had been requested by the Government to publish his works. “No one,” wrote Dumas to Pasteur--“has read Lavoisier with more attention than you have; no one can judge of him better.... The chance which caused me to be born before you has placed me in communication with surroundings and with men in whom I have found the ideas and feelings which have guided me in this work. But, had it been yours, I should have allowed no one else to be the first in drawing the world’s attention to it. It is from this motive, also from a certain conformity of tastes and of principles which has long made you dear to me, that I now ask you to give up a few hours to Lavoisier.” “My dear and illustrious master,” answered Pasteur (July 18, 1865), “in the face of your letter and its expressions of affectionate confidence, I cannot refuse to submit to you a paper which you must promise to throw away if it should not be exactly what you want. I must also ask you to grant me much time, partly on account of my inexperience, and partly on account of the fatigue both mental and bodily imposed on me by the illness of our dear child.” Dumas replied: “Dear friend and colleague, I thank you for your kind acquiescence in Lavoisier’s interests, which might well be your own, for no one at this time represents better than you do his spirit and method,--a method in which reasoning had more share than anything else. “The art of observation and that of experimentation are very distinct. In the first case, the fact may either proceed from logical reasons or be mere good fortune; it is sufficient to have some penetration and the sense of truth in order to profit by it. But the art of experimentation leads from the first to the last link of the chain, without hesitation and without a blank, making successive use of Reason, which suggests an alternative, and of Experience, which decides on it, until, starting from a faint glimmer, the full blaze of light is reached. Lavoisier made this art into a method, and you possess it to a degree which always gives me a pleasure for which I am grateful to you. “Take your time. Lavoisier has waited seventy years! It is a century since his first results were produced! What are weeks and months? “I feel for you with all my heart! I know how heartrending are those moments by the deathbed of a suffering child. I hope and trust this great sorrow will be spared you, as indeed you deserve that it should be.”