Given the following context:

Massenet returned to Paris in 1866. He made a living by teaching the piano and publishing songs, piano pieces and orchestral suites, all in the popular style of the day. Prix de Rome winners were sometimes invited by the Opéra-Comique in Paris to compose a work for performance there. At Thomas's instigation, Massenet was commissioned to write a one-act opéra comique, La grand'tante,  presented in April 1867. At around the same time he composed a Requiem, which has not survived. In 1868 he met Georges Hartmann, who became his publisher and was his mentor for twenty-five years; Hartmann's journalistic contacts did much to promote his protégé's reputation.In October 1866 Massenet and Ninon were married; their only child, Juliette, was born in 1868. Massenet's musical career was briefly interrupted by the Franco-Prussian War of 1870–71, during which he served as a volunteer in the National Guard alongside his friend Bizet. He found the war so "utterly terrible" that he refused to write about it in his memoirs. He and his family were trapped in the Siege of Paris but managed to get out before the horrors of the Paris Commune began; the family stayed for some months in Bayonne, in southwestern France.After order was restored, Massenet returned to Paris where he completed his first large-scale stage work, an opéra comique in four acts, Don César de Bazan (Paris, 1872). It was a failure, but in 1873 he succeeded with his incidental music to Leconte de Lisle's tragedy  Les Érinnyes and with the dramatic oratorio, Marie-Magdeleine, both of which were performed at the Théâtre de l'Odéon. His reputation as a composer was growing, but at this stage he earned most of his income from teaching, giving lessons for six hours a day.

answer the following question:

What is the name of the person whose mentor for twenty-five years was Hartmann? 

Massenet