Subject to the exception hereinafter stated, there is hereby levied and imposed upon all net estates passing by will, devise or under the intestate laws of the State of Alabama, or otherwise, which are lawfully subject to the imposition of an estate tax by the State of Alabama, a tax equal to the full amount of state tax permissible when levied by and paid to the State of Alabama as a credit or deduction in computing any federal estate tax payable by such estate according to the act of Congress in effect, on the date of the death of the decedent, taxing such estate with respect to the items subject to taxation in Alabama. The tax hereby imposed shall not exceed in the aggregate amounts which may by any law of the United States be allowed to be credited against or deducted from such federal estate tax. The estate tax hereby levied shall be levied only so long as and during the time an inheritance or estate tax is enforced by the United States against Alabama inheritances or estates and shall only be exercised or enforced to the extent of absorbing the amount of any deduction or credit which may be permitted by the laws of the United States now existing or hereafter enacted to be claimed by reason thereof as a deduction or credit against such similar tax of the United States applicable to Alabama inheritances or estates. The rates of taxation, the definition of "net estate" and the methods of arriving thereat shall be as provided in such act of Congress.