On the other hand, money easily becomes a 'drug,' as the phrase is, and there is soon too much of it.

The number of accepted securities is limited, and cannot be rapidly increased; if the amount of money seeking these accepted securities is more than can be lent on them the value of money soon goes down.

You may often hear in the market that bills are not to be had, meaning good bills of course, and when you hear this you may be sure that the value of money is very low.
If money were all held by the owners of it, or by banks which did not pay an interest for it, the value of money might not fall so fast.

Money would, in the market phrase, be 'well held.' The possessors would be under no necessity to employ it all; they might employ part at a high rate rather than all at a low rate.

But in Lombard Street money is very largely held by those who do pay an interest for it, and such persons must employ it all, or almost all, for they have much to pay out with one hand, and unless they receive much with the other they will be ruined.