My son has found himself obliged to arrest Prince Cellamare, because they found on his messenger, who was the Abbé Porto-Carrero, letters from the ambassador which revealed a conspiracy against the king and against my son. The ambassador was arrested by two of the Councillors of State. In his secret despatches he warned Alberoni to be very careful not to be on good terms with my son, because as soon as the treaty was signed he meant to poison the little king; the ambassador added that he would see that my son had his hands too full to think of war, for he had brought a number of provinces to promise to revolt; that their party was strong in Paris, and that Alberoni had only to send money and not spare it. I believe the lamester, brother of my daughter-in-law, will be found in this affair. The ambassador has been interrogated by the two Councillors of State, and he admitted, laughing, that he wrote the letters in order to avoid the evils of war, and wanted simply to frighten my son. When they asked him why he had said such infamies of the regent, he replied that he must admit there had been a little poison in his remarks, but that poison was necessary to compose an antidote. What is very strange is that the Maréchal de Noailles, once my son's sub-governor, is implicated in the plot; that is because he is related to that devil incarnate, the Princesse des Ursins, who will pursue my son to the death,--her sole motive being that he thought her too old to wish to be her lover. Cellamare's letters have been printed, so that every one can see the thread of the conspiracy.