Brain-Teaser 501

From The Sunday Times, 10th January 1971 [link]

The grain retailer had with his scales a set comprising the smallest number of weights which, use in any way required, enabled him to weigh loads of every exact number of kilos from 1 to 40 kilos.

One day he mislaid one of his weights and, remembering that the rival shop next door had had an identical set which was no longer in use, he asked to borrow it.

That set too, however, proved to have one of its weights missing, and so it turned out that, even with all the surviving weights in both sets at his disposal, he could weigh only 25 different loads within the required range of 1-40 kilos.

What was the retailers missing weight?