Section 87.
The money belonging to the sinking fund, provided for in the preceding section of this act, shall be by the city council, invested from time to time in interest-bearing bonds of the United States, or of this State, or of Polk County, or of some municipal corporation or school district of this State, or used in purchasing any outstanding bonds of the town, for the payment of which said sinking fund was created; And the council shall have the power to collect any of the securities herein mentioned when due and to collect the interest arising therefrom when said interest shall become due and payable. And the council shall place said interest, when collected, in the sinking fund provided for in this act, and the same shall thenceforth be considered a part of said sinking fund, and shall be kept and used in the manner as hereinbefore provided for the keeping and use of said fund.