Future goods are defined in my textbook as: "1) Goods to be manufactured by the seller. 2) Goods to be aqcuired by the seller [by purchase, gift, succession or otherwise], goods not yet in existence [e.g. lambs to e born]. 3) Things which do exist, but are not yet 'goods' [e.g. minerals not yet extracted but still forming part of the seller's land]. Also defined in s.5(1) SGA.
Future goods are treated as agreements to sell as under s.5(3) SGA they have to become existing goods for property to pass, which means they must be in possession of the seller. s18 rule 1 says that property passes in an unconditional contract for the sale of specific goods that are in a deliverable state as soon as the contract is made. But if a company contracts with a supplier for goods and they are sent by courier (payment on delivery), and company then sells those particular goods in an unconditional contract to a buyer before they are delivered to him - technically if the goods are in transit they are in a deliverable state. BUT company can't pass property to the buyer as he doesn't yet have it. If s18 rule 1 applied the buyer would get property and risk before the company even had property, which doesn't make sense.
Future goods aren't treated as unascertained goods, as soon as they become existing goods the normal rules for specific goods then apply, they just have to become existing goods first.
(This is what I can make out from my notes anyways =] )