17The directors may, if they think fit, receive from any member willing to advance the same all or any part of the moneys uncalled and unpaid upon any shares held by him ; and upon all or any of the moneys so advanced may (until the same would, but for such advance, become presently payable) pay interest at such rate (not exceeding, without the sanction of the company in general meeting, six per cent.) as may be agreed upon between the member paying the sum in advance and' the directors.