32 FAMILIAR LETTERS ON CHEMISTRY. 



We well know that marine plants cannot derive a supply of humus for their 

 nourishment through their roots. Look at the great sea-tang, the Fucus Gigantius: 

 this plant, according to Cook, reaches a height of three hundred and sixty feet, and 

 a single specimen, with its immense ramifications, nourishes thousands of marine 

 animals; yet its root is a small body, no larger than the fist. What nourishment 

 can this draw from a naked rock, upon the surface of which there is no perceptible 

 change? It is quite obvious that these plants require only a hold a fastening to 

 prevent a change of place as a counterpoise to their specific gravity, which is less 

 than that of the medium in which they float. That medium provides the necessary 

 nourishment, and presents it to the surface of every part of the plant. Sea-water 

 contains not only carbonic acid and ammonia, but the alkaline and earthy phos- 

 phates and carbonates required by these plants for their growth, and which we 

 always find as constant constituents of their ashes. 



All experience demonstrates that the conditions of the existence of marine plants 

 are the same which are essential to terrestrial plants. But the latter do not live 

 like sea plants, in a medium which contains all their elements, and surrounds with 

 appropriate nourishment every part of their organs ; on the contrary, they require 

 two media, of which one, namely the soil, contains those essential elements which 

 are absent from the medium surrounding them, that is, the atmosphere. 



Is it possible that we could ever be in doubt respecting the office which the soil 

 and its component parts subserve in the existence and growth of vegetables ? that 

 there should have been a time when the mineral elements of plants were not 

 regarded as absolutely essential to their vitality ? Has not the same circulation been 

 observed on the surface of the earth, which we have just contemplated in the 

 ocean the same incessant change, disturbance, and restitution of equilibrium? 



Experience in agriculture shows that the production of vegetables on a given sur- 

 face increases with the supply of certain matters, originally parts of the soil which 

 had been taken up from it by plants the excrements of man and animals. These 

 are nothing more than matters derived from vegetable food, which in the vital pro- 

 cesses of animals, or after their death, assume again the form under which they 

 originally existed, as parts of the soil. Now, we know that the atmosphere contains 

 none of these substances, and, therefore, can replace none; and we know that their 

 removal from a soil destroys its fertility, which may be restored and increased by a 

 new supply. 



Is it possible, after so many decisive investigations into the origin of the elements 

 of animals and vegetables, the use of the alkalies, of lime and the phosphates, 

 any doubt can exist as to the principles upon which a rational agriculture depends ? 

 Can the art of agriculture be based upon any thing but the restitution of a dis- 

 turbed equilibrium ? Can it be imagined, that any country, however rich and 

 fertile, with a flourishing commerce, which for centuries exports its produce in the 

 shape of grain and cattle, will maintain its fertility, if the same commerce does not 

 restore, in some form of manure, those elements which have been removed from the 

 soil, and which cannot be replaced by the atmosphere? Must not the same fate 

 await every such country which has actually befallen the once prolific soil of Vir- 

 ginia, now in many parts no longer able to grow its former staple productions- 

 wheat and tobacco ? 



In the large towns of England the produce both of English and foreign agricul- 

 ture is largely consumed ; elements of the soil, indispensable to plants, do not 

 return to the fields contrivances resulting from the manners and customs of Eng- 

 lish people, and peculiar to them, render it difficult, perhaps impossible, to collect 

 the enormous quantity of the phosphates, which are daily, as solid and liquid excre- 

 ments, carried into the rivers. These phoshates, although present in the soil in the 

 smallest quantity, are its most important mineral constituents. It was observed 

 that many English fields exhausted in that manner, immediately doubled their 

 produce, as if by a miracle, when dressed with bone-earth imported from the conti- 

 nent. But if the export of bones from Germany is continued to the extent it has 

 hitherto reached, our soil must be gradually exhausted, and the extent of our loss 

 may be estimated, by considering that one pound of bones contains as much phos- 

 phoric acid as a hundred weight of grain. 



The imperfect knowledge of nature, and the properties and relations of matter, 

 possessed by the alchemists, gave rise, in their time, to an opinion that metals as 

 well as plants could be produced from a -seed. The regular forms and ramifications 

 seen in crystals, they imagined to be the leaves and branches of metal plants ; and 

 as they saw the seed of plants grow, producing root, stem, and leaves, and again 

 blossoms, fruits, and seeds, apparently without receiving any supply of appropriate 

 material, they deemed it worthy of zealous inquiry to discover the seed of gold, and 

 the earth necessary for its development. If the metal seeds were once obtained, 



