5. Trade has been spoken of as an agent of exchange. An _instrument_ also is needed. The primitive method of exchange was by barter. That is, by giving the commodity one produces for that which one desires to possess. But this was early found inconvenient. The man who made shoes and wished to exchange some of them for a coat, would not readily find a coat-maker in want of shoes; or if he should, the latter very likely would not want just so many pairs of shoes as would be equal in value to the coat. All other exchanges might be at a similar disadvantage. What is needed is a commodity which will be a _medium_ of exchange—which every one will be willing to receive for any commodity which he has for sale, and which will command anything which he wishes to buy. Such a commodity is usually the main element in the machinery of exchange, and is what constitutes _money_. This instrument in order to meet the want, it is generally believed, must have the following characteristics: 1. Value in the material of which it is made. 2. Uniformity of value throughout the world. 3. Much value in small bulk. 4. Approximate constancy of value. 5. Not readily destructible. 6. Divisibility into small portions which are capable of being reunited. 7. Of universal use. 8. Capable of receiving stamps and marks. Most of these properties are found in gold and silver, if not to such an extent as has been claimed for them, at least so far that they have been the basis of the money of the civilized world. 6. But supplementing in a certain way, and representing these, the instrument of exchange comprises also the large element of _credit_. This consists chiefly of book accounts, promissory notes, bank notes, government notes, bank deposits, checks, drafts, bills of exchange, stocks and bonds. One of the great agencies in modern commerce by which credit is made effectual as a part of the mechanism of exchange is that of _banks_. Banks are institutions which serve to abbreviate and facilitate the business of exchange and to extend and render available the credit of the community. There are four kinds of banks, namely: savings banks, banks of deposit, banks of circulation and issue, and banks of discount. In our modern banking system the last three are generally found in combination, that is, each bank exercises all the functions implied. A savings bank is an institution in which small sums of money are deposited from time to time as they accumulate in the hands of persons of moderate incomes. The depositors are credited with these amounts, and receive a certain, usually not very large, rate of interest in any case, and an additional amount contingently. The bank loans the money thus deposited in large sums to trustworthy persons who can furnish good security, the rate of interest being somewhat higher than that paid to the depositor.