The commission may, after hearing, require public service corporations to carry a proper and adequate depreciation account in accordance with regulations and forms of account it prescribes. It may ascertain and fix the proper and adequate rates of depreciation of the several classes of property for each, and each corporation shall conform its depreciation accounts to the rates so ascertained and fixed, and shall set aside the money so provided for out of earnings and carry such money in a depreciation fund and expend the fund, and the income therefrom, only for the purposes and under rules and regulations, both as to original expenditure and subsequent replacement, as the commission prescribes.