3g FAMILIAR LETTERS ON CHEMISTRY. 



the clay silicates. Many of these, in their natural state, are not acted on by acids, 

 but they become perfectly soluble if heated to redness before the application of the 

 acid. This property belongs to potters' clay, pipe-clay, loam, and many different 

 modifications of clay in soils. In their natural state they may be boiled in con- 

 centrated sulphuric acid, without sensible change ; but if freely burned, as is done 

 with the pipe-clay in many alum manufactories, they dissolve in the acid with the 

 greatest facility, the contained silica being separated like a jelly in a soluble state. 

 Potters' clay belongs to the most sterile kinds of soil, and yet it contains within itself 

 all the constituent elements essential to a most luxurious growth of plants ; but 

 their mere presence is insufficient to secure this end. The soil must be accessible 

 to the atmosphere, to its oxygen, to its carbonic acid ; these must penetrate it, in 

 order to secure the conditions necessary to a happy and vigorous development of 

 the roots. The elements present must be brought into that peculiar state of com- 

 bination which will enable them to enter into plants. Plastic clay is wanting in 

 these properties ; but they are imparted to it by a feeble calcination. 



At Hardwicke Court, near Gloucester, I have seen a garden (Mr. Baker's) con- 

 sisting of a stiff clay, which was perfectly sterile, become, by mere burning, 

 extremely fertile. The operation was extended to a depth of three feet. This was 

 an expensive process, certainly ; but it was effectual. 



The great difference in the properties of burnt and unburnt clay is illustrated 

 by what is seen in brick houses, built in moist situations. In the towns of Flan- 

 ders, for instance, where most buildings are of brick, efflorescences of salts cover 

 the surfaces of the walls, like a white nap, within a few days after they are erected. 

 If this saline incrustation is washed away by the rain, it soon reappears ; and this 

 is even observed on walls which, like the gateway of Lisle, have been erected for 

 centuries. These saline incrustations consist of carbonates and sulphates, with 

 alkaline bases ; and it is well known these act an important part in vegetation. 

 The influence of lime in their production is manifested by their appearing first at 

 the place where the mortar and brick come into contact. 



It will now be obvious to you, that in a mixture of clay with lime, all the condi- 

 tions exist for the solution of the silicated clay, and the solubility of the alkaline 

 silicates. The lime gradually dissolving in water Charged with carbonic acid, acts 

 like milk of lime upon the clay. This explains also the favorable influence which 

 marl (by which term all those varieties of clay rich in lime are designated) exerts 

 upon most kinds of soil. There are marly soils wjiich surpass all others in fertility 

 for all kinds of plants ; but I believe marl in a burnt etate must be far more 

 effective, as well as other materials possessing a similar composition ; as, for 

 instance, those species of limestone which are adapted to the preparation of 

 hydraulic cements for these carry to the soil not only the alkaline bases useful to 

 plants, but also silica in a state capable of assimilation. 



The ashes of coals and lignite are also excellent means of ameliorating the soil, 

 and they are used in many places for this purpose. The most suitable may be 

 readily known by their property of forming a gelatinous mass when treated with 

 acids, or by becoming, when mixed with cream of lime, like hydraulic lime solid 

 and hard as stone. 



I have now, I trust, explained to your satifaction, that the mechanical operations 

 of agriculture the application of lime and chalk to lands, and the burning of 

 clay depend upon one and the same scientific principle : they are means of acce- 

 lerating the decomposition of the alkaline clay silicates, in order to provide plants, 

 at the beginning of a new vegetation, with certain inorganic matters indispensable 

 for their nutrition. 



LETTER XIV. 



MY DBA* SIR t 



I treated, in my last letter, of the means of improving the condition of the soil 

 for agricultural purposes by mechanical operations and mineral agents. I have 

 now to speak of the uses and effects of animal exuviae, and vegetable matters or 

 manures properly so called. 



In order to understand the nature of these, and the peculiarity of their influence 



