THE ART OF CULTURE. 



53 



8'54 parts (Schrader;) and one hundred 

 parts of the stalks of oats, only 4-42 ; the 

 ashes of all these are of the same compo- 

 sition. 



We nave in these facts a clear proof of 

 what plants require for their growth. Upon 

 the same field, which will yield only one 

 harvest of wheat, two crops of barley and 

 three of oats may be raised. 



All plants of the grass kind require sili- 

 cate of potash. Now this is conveyed to 

 the soil, or rendered soluble in it by the irri- 

 gation of meadows. The equiselacece, the 

 reeds and species of cane, for example, 

 which contain such large quantities of sili- 

 ceous earth, or silicate of potash, thrive 

 luxuriantly in marshes, in argillaceous soils, 

 and in ditches, streamlets, and other places 

 where the change of water renews con- 

 stantly the supply of dissolved silica. The 

 amount of silicate of potash removed from 

 a meadow in the form of hay is very con- 

 siderable. We need only call to mind the 

 melted vitreous mass found on a meadow 

 between Manheim and Heidelberg after a 

 thunder-storm. This mass was at first sup- 

 posed to be a meteor, but was found on ex- 

 amination (by Gmelin) to consist of silicate 

 of potash ; a flash of lightning had struck a 

 stack of hay, and nothing was found in its 

 place except the melted ashes of the hay. 



Potash is not the only substance necessary 

 for the existence of most plants ; indeed it 

 has been already shown that the potash may 

 be replaced in many cases by soda, magne- 

 sia, or lime; but other substances besides 

 alkalies are required to sustain the life of 

 plants. 



Phosphoric acid has been found in the 

 ashes of all plants hitherto examined, and 

 always in combination with alkalies or alka- 

 line earths.* Most seeds contain certain 

 quantities of phosphates. In the seeds of 

 different kinds of corn particularly, there is 

 abundance of phosphate of magnesia. 



Plants obtain their phosphoric acid from 

 the soil. It is a constituent of all land capa- 

 ble of cultivation, and even the heath at 

 Luneburg contains it in appreciable quan- 

 tity. Phosphoric acid has been detected 



* Professor Connall was lately kind enough to 

 show me about half an ounce of a saline powder, 

 which had been taken from an interstice in the 

 body of a piece of teak timber. It consisted es- 

 sentially of phosphate of lime, with small quan- 

 tities of carbonate of lime and phosphate of mag- 

 nesia. This powder had been sent to Sir David 

 Brewster from India, with the assurance that it 

 was the same substance which usually is found in 

 the hollows of teak timber. It has long been 

 known that silica, in the form of tabasheer, is se- 

 creted by the bamboo ; but I am not aware that 

 phosphates have been found in the same condi- 

 tion. Without more precise information, we must 

 therefore suppose that they are left in the hollows 

 by the decay of the wood. Decay is a slow pro- 

 cess of combustion, and the incombustible ashes 

 must remain after the organic matter has been 

 consumed. But if this explanation be correct, the 

 wood of the teak-tree must contain an enormous 

 quantity of earthy phosphates. ED. 



also in all mineral waters in which its pre- 

 sence has been tested ; and in those in 

 which it has not been found it has not been 

 sought for. The most superficial strata of 

 the deposits of sulphuret of lead (galena) 

 contain crystallised phosphate of lead (green- 

 lead ore;) clay-slate, which forms extensive 

 strata, is covered in many places with crys- 

 tals of phosphate of alumina (Wavellite ;) 

 all its fractured surfaces are overlaid with it. 

 Phosphate of lime Apatite) is found even 

 in the volcanic boulders on the Laacher 

 See in the Eifel,near Andernach.* 



The soil in which plants grow furnishes 

 them with phosphoric acid, and they in turn 

 yield it to animals, to be used in the forma- 

 tion of their bones, and of those constituents 

 of the brain which contain phosphorus. 

 Much more phosphorus is thus afforded to 

 the body than it requires, when flesh, bread, 

 fruit, and husks of grain are used for food, 

 and this excess is eliminated in the urine 

 and the solid excrements. We may form 

 an idea of the quantity of phosphate of 

 magnesia contained in grain, when we con- 

 sider that the concretions in the caecum of 

 horses consist of phosphate of magnesia 

 and ammonia, which must have been ob- 

 tained from the hay and oats consumed as 

 food. Twenty-nine of these stones were 

 taken after death from the rectum of a horse 

 belonging to a miller, in Eberstadt, the total 

 weight of which amounted to 3 Ibs. ; and 

 Dr. F. Simon has latelv described a similar 

 concretion found in tne norse ol a carrier, 

 which weighed l Ib. 



It is evident that the seeds of corn could 

 not be formed without phosphate of magne- 

 sia, which is one of their invariable con- 

 stituents; the plant could not under such 

 circumstances reach maturity. 



Some plants, however, extract- other mat- 

 ters from the soil besides silica, potash, and 

 phosphoric acid, which are essential con- 

 stituents of the plants ordinarily cultivated .f 

 These other matters, we must suppose, 

 supply, in part at least, the place and per- 

 form the functions of the substances just 

 named. We may thus regard common salt, 

 sulphate of potash, nitre, chloride of potas- 

 sium, and other matters, as necessary con- 

 stituents of several plants. 



Clay-slate contains generally small quan- 

 tities of oxide of copper; and soils formed 

 from micaceous schist contain some metallic 

 fluorides. Now, small quantities of these 

 substances also are absorbed into plants, al- 

 though we cannot affirm that they are ne- 

 cessary to them. 



It appears that in certain cases flouride of 

 calcium may take the place of phosphate 

 of lime in the bones and teeth; at least it is 

 impossible otherwise to explain its constant 

 presence in the bones of antediluvian ani- 

 mals, by which they are distinguished from 



* See the analyses of soils in the Appendix, 

 t For more minute information regarding soilg 

 see the supplementary chapter at the end of Part 1, 



