In other cases the fact that the future goods have been acquired or manufactured by the seller, or that other unascertained goods have become ascertained, as the case may be, may do no more than make it possible for the property to pass.
It is then usually necessary to find some evidence of the intention of the parties as to the time at which the property is to pass, as, for instance, (1) the happening of a specified event which identifies the goods, and on the happening of which it is agreed that the property shall pass, or (2) the unconditional appropriation of the goods, subsequently to the contract and as the subject matter thereof, with the assent of both parties. Cf. 25 Halsbury, Laws of England, pp. 167 ff., where the subject is worked out in detail.
The rule as to the appropriation of the goods to the contract is stated in the Sale of Goods Act (Ont. s. 20; U. K. s. 18) as follows:
20. Unless a different intention appears, the following are rules for ascertaining the intention of the parties as to the time at which the property in the goods is to pass to the buyer:
(e) Rule 5. - (i) Where there is a contract for the sale of unascertained or future goods by description, and goods of that description and in a deliverable state, are unconditionally appropriated to the contract, either by the seller with the assent of the buyer, or by the buyer with the assent of the seller, the property in the goods thereupon passes to the buyer. Such assent may be expressed or implied. and may be given either before or after the appropriation is made. (ii) Where, in pursuance of the contract, the seller delivers the goods to the buyer or to a carrier or other bailee (whether named by the buyer or not), for the purpose of transmission to the buyer, and does not reserve the right of disposal, he is deemed to have unconditionally appropriated the goods to contract.
"Under a contract for sale of chattels not specific the property does not pass to the purchaser unless there is afterwards an appropriation of the specific chattels to pass under the contract, that is, unless both parties agree to the specific chattels in which the property is to pass, and nothing remains to be done in order to pass it."
Mirabita v. Imperial Ottoman Bank, 1878, 3 Ex. D. 164, at p. 172; cf. Pignataro v. Gilroy,  1 K.B. 459 (assent of buyer implied from failure to reply to notice) ; Mason & Risch v. Christner, 1918, 44 O.L.R. 146, 46 D.L-R. 710 (no appropriation by seller with consent of buyer), S.C, 1920, 48 O.L.R. 8, 54 D.L.R. 653; Sells v. Thomson, 1914, 19 B.C.R. 400, 17 D.L.R. 737.
An authority to appropriate is presumed where, by the terms of the contract, one party is to do with reference to the_ goods some act or thing which cannot be done until the goods are"appropriated. When the party authorized has determined his election by doing such act or thing, the appropriation is finally made.
25 Halsbury, Laws of England, p. 169, citing Blackburn, Contract of Sale, 1st ed., p. 128, 3rd ed., p. 138; Fragano v. Long, 1825, 4 B. & C. 219 (insuring and despatch of goods by seller upon buyer's instructions). The delivery of parts of a machine may be a sufficient appropriation to cause the property to pass, notwithstanding that there are supplemental provisions in the contract for the erection of the complete machine by the seller on the buyer's premises.
Pritchett, etc., Co. v. Currie,  2 Ch. 515. Where by the terms of a contract for unascertained goods, the seller agrees to deliver the goods at a particular place, and no intention appears in the contract that the property shall pass previously to such delivery, the property does not pass unless and until delivery is made accordingly.
Calcutta and Burmah Steam Navigation Co. v. De Mat-tos, 1863, 32 L.J.Q.B. 322, at pp. 335, 328.
The reason for the rule is that until delivery the appropriation is not_complete as till then the seller may change his mind. See 25 Halsbury, Laws of England, p. 174, where some familiar examples are given. In the United States a similar rule is stated in the Uniform Sales Act, but is not limited, as it should be, to contracts for unascertained goods.