The holder of a mortgage is at a great disadvantage in regard to the changing value of real estate. If the value of the property upon which he holds a mortgage increases, the additional value enhances the security of the loan, but does not add to the principal which he has invested, while if the value of the property diminishes, not only is the security proportionately lessened, but if the impairment be great, the holder is frequently compelled to take over the property and may suffer loss of principal. In other words, he receives no direct benefit from an increase in the value of the property, but has to stand the larger part of the risk of a decline in its value. This is not the case with investments represented by negotiable securities subject to changing market quotations. All such securities, railroad bonds for example, are acted on equally by changes in the value of the property which secures them. Except for the influences of money-market conditions, railroad bonds advance with an increase in the value of the property and decline with a decrease in its value. Well-selected bonds usually increase in value with time, and all such increase goes directly to the benefit of the holder. The failure of real-estate mortgages to respond similarly to changes in the value of property places the holder of a mortgage at a great disadvantage. Owing to this characteristic, real-estate mortgages should be purchased only when general conditions in the real-estate market are distinctly favorable. Not only should the purchaser of a mortgage have sufficient margin of security in the particular piece of property upon which he is loaning money, but he should also be satisfied that general real-estate values are relatively low, that there has been no undue speculation, and that conditions favor an advance rather than a decline in real-estate prices.