As manures are liable to considerable fluctuations in price, the value attached to each of their constituents ought to be varied with the state of the market; but it is obviously impossible for the farmer to watch the changes in price with such minuteness as to enable him to do this, and it is much more convenient, as well as safer, to adopt a fixed average, which can be used with reasonable accuracy at all times. The fact is, that this system of valuation is only an approximation to the truth; and if absolute accuracy were aimed at, it would be necessary to vary the estimates, not only at different times, but at different localities at the same time, and to some extent also according to the kind of manure. The price of soluble phosphates more especially, fluctuates to a great extent, being practically fixed by each manufacturer according to the facilities which his position or command of raw material offer for producing them at a low rate. We thus find that when made from bones alone, the cost of that substance is not unfrequently as high as L40 per ton, and when bone-ash alone is used it is sometimes as low as L20. Such extreme differences, of course, cannot be taken into account in the system of valuation adopted, where all that can be done is to take average values, which, when applied to average samples, ought to bring out their value. The data which have already been given regarding the price of the individual constituents of manures can be applied to the determination of the value of any mixture in two different ways by means of the subjoined table:-- +----------------------------+-----------------+-------------------+ | | Price per Ton. | Per cent per Ton. | +----------------------------+-----------------+-------------------+ | Ammonia | L60 0 0 | L0 12 0 | | Insoluble phosphates | 7 0 0 | 0 1 5 | | Do. in phosphatic guanos | 10 0 0 | 0 2 0 | | Soluble phosphates | 30 0 0 | 0 6 0 | | Biphosphate of lime | 46 16 0 | 0 9 4-1/2 | | Alkaline salts | 1 0 0 | 0 0 2-4/10 | | Sulphate of lime | 1 0 0 | 0 0 2-4/10 | | Potash | 20 0 0 | 0 4 0 | | Nitrate of soda | 16 0 0 | 0 3 2-1/2 | | Organic matter | 0 10 0 | 0 0 1-1/4 | +----------------------------+-----------------+-------------------+ Supposing it be desired to calculate the value of a manure by the first column, it is obvious that if we suppose 100 tons to be purchased, the per centages of the different constituents shewn in the analysis will give the number of tons of each contained in 100 tons of the mixture, and, selecting the analysis of the superphosphate given in a previous page, we proceed in the calculation as follows:--