Point--As we already know, Jekyll is already infected by Hyde as Hyde lives within him, therefore causing his language to be extravagant. However, Lanyon's language is scientific and precise (before the shock) as he is a medically trained doctor-Duality. Also, he uses very little imagery and gives a clear, concise and an accurate account of what he witnesses. He says:

Evidence--      "...began to effervesce audibly, and to throw off small fumes of vapor..."

However, after his shock, his language changes from an articulate man of science to vague and cryptic (secrecy). This shows duality. Lanyon says about Jekyll:

"like a man restored from death"

Explanation--This brilliant quote effectively allows the reader to imagine the state of Jekyll after taking the potion from being Hyde. Not only does I show this, it also shows the imagery of Hyde spreading as a viral disease. Lastly, Lanyon's language has clearly taken a big shift, as said above, from precise to ambiguous.

Technique & Zoom--The technique used here is a simile and dark imagery. Lanyon says "restored"- meaning back to life "from death". This shows duality. But it also shows that Jekyll is the one whos being given life "restored" when he comes back from Hyde-"from death". This shows that Lanyon is foreshadowing his own death as well as Jekyll's (dark imagery) because he is calling Hyde- death, who has a relationship with Jekyll. And now that Lanyon had a relationship with Hyde too, both of them are going to die.

Effect upon reader--Here the reader feels sorry for Lanyon but furious with Jekyll as his experiments lead to the death of so many people around him.