Equipment bonds vary in point of convertibility. The reader will remember from the description above that equipment bonds are usually issued in serial form, with instalments maturing semiannually from six months to ten years. By confining purchases to the shorter maturities, say within two or three years, a high degree of convertibility may usually be obtained because the short maturities are greatly sought by banks and other financial institutions which regard equipment bonds in much the same light as merchant's paper or time loans secured by collateral. At a price equivalent to the rate which the best commercial paper commands, there is always a good demand from the banks. Many banks prefer equipment bonds to loans or paper on account of their greater convertibility. As the length of maturity increases, the degree of convertibility generally decreases, because the chief demand for the longer dates comes from insurance companies, which do not, in the aggregate, constitute as great a demand as the banks. When the demand from private investors increases, as it undoubtedly will when they become more familiar with the desirable points of these issues, all maturities will probably possess ready convertibility. In the same way, equipment bonds vary as to stability of market price. Compared with other classes of railroad issues, equipment bonds are all relatively stable, but the stability is especially marked in the shorter maturities. Equipment bonds possess little prospect of appreciation in value. The attentive reader who has carefully followed the foregoing description of equipment bonds, may have noticed a special adaptability on their part to the requirements of a business surplus. Broadly speaking, for such investment, a security is required which will combine perfect safety of principal and interest, a good rate of income, ready convertibility into cash, and unyielding stability of market price. The necessity for insistence upon these requirements in the investment of a business surplus will appear upon a moment's reflection. Safety is required in all forms of investment, but is particularly important in the handling of business funds; a good rate of income is always desirable; convertibility is necessary for a business surplus so that the reserve funds may be converted into cash at any time; and it is of the utmost importance that the security should not shrink materially in quoted price, no matter what changes may take place in financial and business conditions, so that if the need should arise for realizing on the reserve fund, it would be found unimpaired in amount. As explained in a former chapter, this point can not be covered by the selection of securities perfectly safe as to principal and interest, but only by the purchase of short-term obligations.