Why does mond deny johns request to go to the island with helmhotz? What is mond hoping?

In Chapter Eighteen, Helmholtz and Bernard go to visit John, who is vomiting in his room. When they ask him what is wrong, he replies, "I ate civilization... It poisoned me." John tells the two men that he visited Mustapha Mond that morning and asked if he could join them on the island. Mustapha refused his request, indicating that he wanted to continue the experiment of reconciling John to civilization.