what relevance do chapters 9 and 10 add to Dr. Jekyll and Mr. Hyde?

Finally, in this chapter, the nature of Jekyll's experimentation and his dual existence as Jekyll/Hyde is revealed. Lanyon's description of that fateful night reveals the otherwise mysterious relationship between Hyde and Jekyll. Through this letter, Stevenson finally directly embraces the supernatural theme of the novel, and reveals the horror Jekyll's transformation inflicted on Lanyon.

Lanyon includes many details in his letter, even noting the colors of the various vial contents in Jekyll's drawer. However, although he is very detailed in what he witnessed that night, Lanyon does not provide an explanation of how such a transformation could occur, or how Jekyll's scientific experiments advanced and progressed to this point. Lanyon purposely does not include this information, as he simply finds it to offensive to write about. Clearly, this is important information, but Lanyon refuses to discuss it, just as he refused to share this information immediately after witnessing it. Instead, Lanyon forced society to wait until Jekyll's death or disappearance and his own death before the truth would be revealed.

The novel contains many other silences, such as the lack of description of Hyde's face, and mutual agreement between Enfield and Utterson to avoid speaking of Jekyll's apparent seizure and suffering at the window. Lanyon's silence here is a reflection of how intensely he wishes to reject Jekyll's work. We learned earlier that Jekyll and Lanyon, the rationalist, had a falling out over the legitimacy of pursuing mystical science. Now, having been proved wrong, Lanyon refuses to accept or acknowledge scientific achievement or work that is entirely contrary to his perspective. So offensive is the shock of Jekyll's work, that Lanyon is affected physically. He grows weak, internalizes his pain, protects the truth, and eventually dies as a result of the shock of witnessing Hyde's transformation into Jekyll.

This fleshes out many details that we have waited for thus far in the story. The relationship and subsequent lack of it between Lanyon and Jekyll. There is also the revelation that Jekyll and Hyde are technically the same person.

Thank you Aslan. Much appreciated!!!!

You are welcome!