51. (1) Notwithstanding anything to the contrary contained in any law, no child shall be sentenced to death, transportation or imprisonment: Provided that when a child is found to have committed an offence of so serious a nature that the Court is of opinion that no punishment, which under the provisions of this Act it is authorised to inflict, is sufficient or when the Court is satisfied that the child is of so unruly or of so depraved character that he cannot be committed to a certified institute and that none of the other methods in which the case may legally be dealt with is suitable, the Court may sentence the child to imprisonment or order him to be detained in such place and on such conditions as it thinks fit: Provided further that no period of detention so ordered shall exceed the maximum period of punishment to which the child could have been sentenced for the offence committed: Provided further that at any time during the period of such detention the Court may, if it thinks fit, direct that in lieu of such detention the youthful offender be kept in a certified institute until he has attained the age of eighteen years. (2) A youthful offender sentenced to imprisonment shall not be allowed to associate with adult prisoners.