The French Wikipedia article about Les dieux ont soif (The Gods Are Athirst), a novel by Anatole France, contains the following quote attributed to Joseph Conrad:
« C’est un grand analyste d’illusions. Il en pénètre et en sonde les plus secrets replis comme s’il s’agissait de réalités faites de substances éternelles. Et c’est en quoi consiste son humanité : elle est l’expression de sa profonde et inaltérable compassion. »
— Joseph Conrad [réf. nécessaire]
Since I couldn't find Conrad's original words, which were presumably in English, here is my own translation:
He is a great analyst of illusions. He pierces and probes their most secret recesses as if they were realities made out of eternal substances. And this is what his humanity consists in: the expression of his deep and unalterable compassion.
The quote has been reproduced on countless websites, but, as far as I can tell, always without a source. Some sites, e.g. online-litterature.com add La Révolte des anges, but that is a different novel by Anatole France and not one of Joseph Conrad's works. Based on this, it is possible that the Wikipedia article about Les dieux ont soif actually uses a quote about a different novel.
I have tried to find comments by Conrad on Anatole France on Google Books and I found something in The Complete Works of Joseph Conrad and in Joseph Conrad: The Complete Collection. The context is a discussion of Anatole France's novella Putois:
All this is told with the wit and the art and the philosophy which is familiar to M. Anatole France's readers and admirers. For it is difficult to read M. Anatole France without admiring him. He has the princely gift of arousing a spontaneous loyalty, but with this difference, that the consent of our reason has its place by the side of our enthusiasm. He is an artist. As an artist he awakens emotion. The quality of his art remains, as an inspiration, fascinating and inscrutable; but the proceedings of his thought compel our intellectual admiration. (...) The vision of M. Anatole France, the Prince of Prose, ranges over all the extent of his realm, indulgent and penetrating, disillusioned and curious, finding treasures of truth and beauty concealed from less gifted magicians. (...)
It would be possible to quote more passages about Anatole France, but none of those I have access to come close to the wording quoted in Wikipedia. What is the actual source of that quote and which of Anatole France's works is Conrad discussing in that context?