6It is hereby declared that, where any business or trade is confined to the management of any particular assets, but power exists to substitute other assets for those particular assets or any of them, such a substitution shall not be deemed, for the purposes of Part III of this Act, to constitute a change of ownership of the business ; but, where any such substitution has been carried out- by the sale of assets and the purchase of other assets, the capital of the trade or business shall be taken to be increased or decreased, as the case may be, only by the amount of the difference between the price of the - assets purchased and the price obtained for the assets sold, and the capital representing the assets purchased shall be estimated on the same basis for all the purposes of Part III of this Act.