Whenever any city of the second class of this commonwealth shall hereafter acquire, by purchase, donation, condemnation or otherwise, any real estate for public park purposes, such city is hereby authorized and empowered to set aside such part or parts of said real estate as it may elect so to set aside, for a building site or sites for technological schools, or¹ institutions for the promotion of arts and sciences, and to permit the use of such site or sites for the purpose of establishing and maintaining thereon such schools and institutions: Provided, however, That no school or institution shall be placed on any plot of ground acquired for public park purposes, which does not exceed five acres in area, unless it immediately adjoins and forms part of a larger tract of ground used for public park purposes: And provided also, That this act shall not be construed so as to permit the establishment or maintenance, in a public park, of any school or institution in which such city shall not have a voice in the management.