218 PROCEEDINGS OF THE CHEMICAL DEPARTMENT. 



two substances which are the main fertilising constituents of 

 the manures employed exist in these guanos in very different 

 proportions from those in which they are contained in the 

 artificial mixtures. The Peruvian guano supplies less phosphoric 

 acid, but more than twice as much ammonia as the mixtures, 

 and the Bolivian guano more than twice as much phosphoric 

 acid, but only a trifling quantity of ammonia. The phosphoric 

 acid in the two guanos is, of course, insoluble in water, with the 

 exception of that small quantity which is found in the alkaline 

 salts. In the mixtures only that which is soluble is taken into 

 account. JSTo doubt, it may be urged that the insoluble phos- 

 phoric acid must here produce some effect ; but this can cer- 

 tainly not be the case with the coprolites ; and even with the dis- 

 solved guano, I believe it will be found as a rule, that in presence 

 of a large amount of soluble phosphates, that portion which is 

 left undissolved has little effect on the crop to which it is applied, 

 though it no doubt tells to some extent on those which follow. 



In weighing out the manures, all the precautions described in 

 my report of last year were adopted. Each manure was taken 

 from the sacks, and the whole well mixed, so that there might be 

 no want of uniformity. The quantity of manure for each in- 

 dividual plot was separately weighed ; and where two substances 

 had to be mixed, this was done by weighing each separately, 

 and stirring them together with the hand in a large basin until 

 they were thoroughly mixed. 



Along with the manures instructions for sowing, similar to 

 those issued in the previous year, were sent to each experi- 

 menter. These I reproduce here, because they contain some 

 precautions which experience showed to be necessary, and they 

 serve also to indicate the exact nature of the experiments. 



Instructions for Laying Out the Plots and Sowing the Manures 

 for the Field Experiments of Season 1867. 



1. Select a portion of the field in which the soil is as uniform 

 as possible, avoiding headlands. 



2. At five or six different places dig a small trench 10 inches 



deep, and from the side of it take a 

 slice with the spade about 3 or 4 

 inches thick, as represented here. 

 Mix the whole in a barrow, and 



take from this 5 or 6 lbs. as a sample of the soil. Go down 

 other 10 inches, and take a similar sample of the subsoil. Small 

 bags and labels for these are sent along with the manures. 



3. Let the drills in all cases be 27 inches wide. 



4. Fix a stake in the corner of the space selected for the ex- 

 periments, and having counted the number of drills required for 

 the plots, fix at the opposite end another stake, so that the line 



