FAMILIAR LETTERS ON CHEMISTRY. 47 



may be sufficient to remind you of the fact, that the blood of man and animals, 

 beside common salt, always contains alkaline and earthy phosphates. If we burn 

 blood and examine the ashes which remain, we find certain parts of them soluble 

 in water, and others insoluble. The soluble parts are, common salt and alkaline 

 phosphates ; the insoluble consist of phosphate of lime, phosphate of magnesia, and 

 oxide of iron. 



These mineral ingredients of the blood without the presence of which in the 

 food the formation of blood is impossible both man and animals derive, either 

 immediately or mediately, through other animals, from vegetable substances used 

 as food ; they had been, constituents of vegetables, they had been parts of the soil 

 upon which the vegetable substances were developed. 



If we compare the amount of the phosphates in different vegetable substances 

 "with each other, we discover a great variety, while there is scarcely any ashes of 

 plants altogether devoid of them, and those parts of plants which experience has 

 taught us are the most nutritious, contain the largest proportion. To these belong 

 all seeds and grain, especially the varieties of bread-corn, peas, beans, and lentils. 



It is a most curious fact that, if we incinerate grain or its flour, peas, beans, and 

 lentils, we obtain ashes, which are distinguished from the ashes of all other parts 

 of vegetables by the absence of alkaline carbonates. The ashes of these seeds, when 

 recently prepared, do not effervesce with acids; their soluble ingredients consist 

 solely of alkaline phosphates, the insoluble parts of phosphate of lime, phosphate 

 of magnesia, and oxide of iron; consequently, of the very same salts which are 

 contained in blood, and which are absolutely indispensable to its formation. We 

 are thus brought to the further indisputable conclusion, that no seed suitable to 

 become food for man and animals can be formed in any plant without the presence 

 and co-operation of the phosphates. A field, in which phosphate of lime, or the 

 alkaline phosphates form no part of the soil, is totally incapable of producing grain, 

 peas, or beans. 



An enormous quantity of these substances, indispensable to the nourishment of 

 plants, is annually withdrawn from the soil and carried into great towns, in the 

 shape of flour, cattle, et cetera. It is certain that this incessant removal of the 

 phosphates must tend to exhaust the land and diminish 'its capability of producing 

 grain. The fields of Great Britain are in a state of progressive exhaustion from 

 this cause, as is proved by the rapid extension of the cultivation of turnips and 

 mangel-wurzel plants which contain the least amount of the phosphates, and there- 

 fore require the smallest quantity for their development. These roots contain 80 

 to 92 per cent, of water. Their great bulk makes the amount of produce fallacious, 

 as respects their adaptation to the food of animals, inasmuch as their contents of the 

 ingredients of the blood that is, of substances which can be transformed into 

 flesh stands in a direct ratio to their amount of phosphates, without which neither 

 blood nor flesh can be formed. 



Our fields will become more and more deficient in these essential ingredients of 

 food, in all localities where custom and habits do not admit the collection of the 

 fluid and solid excrements of man, and their application to the purposes of agricul- 

 ture. In a former letter I showed you how great a waste of phosphates is 

 unavoidable in England, and referred to the well known fact, that the importation 

 of bones restored in a most admirable manner the fertility of the fields exhausted 

 from this cause. In the year 1827, the importation of bones for manure amounted 

 to forty thousand tons, and Huskisson estimated their value to be from one hundred 

 thousand to two hundred thousand pounds sterling. The importation is still greater 

 at present, but it is far from being sufficient to supply the waste. 



Another proof of the efficacy of the phosphates in restoring fertility to exhausted 

 land is afforded by the use of the guano a manure which, although of- recent 

 introduction into England, has found such general and extensive application. 



We believe that the importation of one hundred weight of guano is equivalent to 

 the importation of eight hundred weight of wheat the hundred weight of guano 

 assumes, in a time which can be accurately estimated, the form of a quantity of 

 food corresponding to eight hundred weight of wheat. The same estimate is appli- 

 cable in the valuation of bones. 



If it were possible to restore to the soil of England and Scotland the phosphates 

 which during the last fifty years have been carried to the sea by the Thames and 

 the Clyde, it would be equivalent to manuring with millions of hundred weights of 

 bones, and the produce of the land would increase one-third, or, perhaps, double 

 itself, in five or ten years. 



We cannot doubt that the same result would follow, if the price of the guano 

 admitted the application of a quantity to the surface of the fields, containing aa 

 much of the phosphates as have been withdrawn from them in the same period. 



