Although sound in theory, accelerated aging tests are, at this time, of limited usefulness. The reason is that conditions of storage, which vary widely, have a considerable influence on the degree of permanence; also, it is difficult to verify empirically the accuracy of such tests except by experiments conducted over a number of years. Such tests have actually been made, although to a limited extent. It is known, for example, that the strength of paper tends to diminish in storage, and experiments have indicated that the FOLDING ENDURANCE declines to a significantly greater extent than such other properties as tensile, or tearing, strength; consequently, folding endurance tests conducted subsequent to accelerated aging may well provide a good indication of a general loss of strength. In addition, as to the rate of deterioration, the effect of heat is very much like that of natural aging under average conditions; therefore, it is probably reasonable to assume that heat affords a practical means of accomplishing accelerated aging. (18 , 62 , 144 )