320 Proceedings of the British Association. . 
mode of restoring to the land its original fertility. If the theory 
suggested by the author be the true one, it will follow, that the 
proper remedy would be, not to discontinue the use of the ni- 
trates, but by the application of bone manure, &c. at intermediate 
periods, to restore to the land those other ingredients which had 
been abstracted from it in too large a quantity. To determine 
what materials are wanting, and in what proportions they ought 
to be applied ; (independently of the empirical plan of ascertain- 
ing, by repeated trials, the substances, which, by their addition 
succeed best in remedying the deficiency,) two methods present 
themselves. The first, a difficult one, is to learn by a minute 
analysis of the soil, whether the ingredients which the crop re- 
quires are actually present, and to add of these a quantity equal 
to that which the intended crop is calculated to contain. The 
second, a more practical scheme, is to estimate in the first place, 
how much of these substances exists in the crop taken off the 
ground, and then to add to it at least an equivalent quantity of 
manure. ‘The Doctor suggested, that in farming: establishments, 
a kind of book-keeping should be undertaken, on this principle : 
a debtor and creditor account being made out of the quantity of 
nitrogen, of earthy phosphates, of alkali, &c. abstracted in the 
form of crop, and restored in that of manure each year. He 
concluded by specifying certain points relative to this subject 
which require further investigation. 1. To confirm or disprove 
his theory, with respect to the operation of the nitrates, by deter- 
mining whether they actually diminish. in quantity, and finally 
disappear after several. successive crops have been grown upon 
land impregnated with these salts.. 2. Whether the same applies 
to common salt and other mineral manures as to the nitrates, or 
whether any of them act directly as stimuli. 3. More extended 
and exact data relative to the amount of alkaline and earthy salts, 
and of nitrogen present in the various crops cultivated by the 
farmer, as well as in the manures he employs. 
A practical method of determining the quantity of real Indige 
in the Indigo of Commerce, by Dr. Samuel L. Dana, of Lowell, 
Mass. U.S.A. Dr..D. directs that 10 grains of indigo reduced 
to an impalpable. powder should be boiled in a Florence flask a 
few minutes, in 2402. of a solution of carbonate of soda, mak- 
ing 30° to 35° on 'Twaddel’s hydrometer; then add 8 grains of 
crystals of muriate of tin, and boil for half. an hour; a beautiful 
