ANNUAL OF SCIENTIFIC DISCOVERY. 



bility of soils generally, so infinitely varied was their composition ; 

 but it would be useful to give an illustration on this point. Taking 

 the average of forty-two analyses of fourteen soils, of very various 

 descriptions, it was estimated that it would require, of ordinary rota- 

 tion with home manuring, and selling only corn and meat, about two 

 thousand years to exhaust the potash, about one thousand years to 

 exhaust the phosphoric acid, and about six thousand years to exhaust 

 the silica, found to be soluble in dilute hydrochloric acid, reckoning 

 the soil to be one foot deep. Many soils doubtless had a composition 

 inferior to that here supposed. In a large proportion, however, the 

 amounts of the constituents assumed to be soluble in dilute acid would 

 probably be available for plants before the expiration of the periods 

 mentioned ; whilst in a large proportion there would still be further 

 stores eventually available, within a greater or less depth from the 

 surface. But in practice the exhaustion was really by no means so 

 great as supposed in the above illustrations. Where there was no 

 import of cattle food, or artificial (as town) manures, the sale of min- 

 eral constituents in corn and meat would be much less than were 

 taken in the authors' estimates. Those amounts of exports from the 

 farm could only be reached when cattle food, or direct manures, were 

 purchased by the farmer ; and wherever these courses were pursued 

 judiciously, there was always much more phosphoric acid (the most 

 easily-exhausted constituent) brought upon the land than could be 

 exported in the increase of produce obtained. In such cases, in 

 many soils, potash was more likely to become deficient. Then, again, 

 a not inconsiderable portion of the refuse of our towns was derived 

 from imported food, or other matters not obtained from our own cul- 

 tivated land ; whilst by no means the whole of it reached the sewers, 

 and thence our rivers. In conclusion, whilst the authors believed 

 that modern practices did not tend to exhaustion in anything like the 

 degree that had been supposed by some, they would nevertheless in- 

 sist upon the importance of applying to agricultural purposes as much 

 as possible of the valuable manuring matters of our towns. It was at 

 the same time, certain that if these were to be diluted with water in 

 the degree recognized under the present system, they could then, un- 

 less in'cxceptional cases, be only applicable on the large scale to grass 

 land; and, so far as this was the case, they would, of course, not di- 

 rectly contribute to the restoration to the land under tillage of the 

 mineral constituents sent from it in its produce of corn and meat. 



EXTRACTION OF BUTTER. 



We herewith give the results of a series of experiments made by 

 M. Barral, on the above subject, as reported in u Cosmos" (Paris). 

 The time required for the formation of butter varies considerably 

 with the temperature. At 53 Fah., about ten times more time is 

 required than at 68 ; at 86 the time needed is about one-half less 

 than at 68. Another remarkable fact is, that when the temperature 

 of the churning is too elevated, the yield of butter diminishes very 

 considerably. The most suitable temperature when milk is operated 

 on is between 64 and 68. The loss is much less when cream^ is 

 churned instead of milk. The most suitable temperature for obtain- 



