When does the mood change in "The Monkey's Paw"?

The mood changes from one of simple domestic harmony to one of sorrow and dread right after Part II of the story ends with the announcement by the man from Maw and Meggins that the company is awarding Herbert's parents the sum of two hundred pounds as compensation for the loss of their son in a factory accident. Part II ends with one sentence.

Unconscious of his wife's shriek, the old man smiled faintly, put out his...

The mood changes from one of simple domestic harmony to one of sorrow and dread right after Part II of the story ends with the announcement by the man from Maw and Meggins that the company is awarding Herbert's parents the sum of two hundred pounds as compensation for the loss of their son in a factory accident. Part II ends with one sentence.

Unconscious of his wife's shriek, the old man smiled faintly, put out his hands like a sightless man, and dropped, a senseless heap, to the floor.

When Part III opens, we feel the coldness in the little house and an emptiness left by Herbert's permanent absence.

In the huge new cemetery, some two miles distant, the old people buried their dead, and came back to a house steeped in shadow and silence. 

All of Part III is entirely different in mood from the first two parts of the story. In the final part the couple seem much older. They have lost the happy-go-lucky young lad who provided all the interest and amusement in their lives. Now their lives seem empty and their house seems haunted. When Mrs. White gets the fantastic idea of having her husband wish for Herbert to come back to life, the mood becomes worse than sad. It starts to grow ominous. They are trafficking with supernatural forces they do not understand. If they had to lose their son Herbert in order to obtain the two hundred pounds they wished for--then what would they have to lose if their second wish were granted? The uncanniness increases until it seems like a mixture of madness on the part of the mother and pure horror on the part of the father. The reader too is infected with their emotions and is vastly relieved when Mr. White thinks of uttering his third and final wish and makes that dreadful knocking stop.