208 SOIL ITS ANALYSIS. 



and evaporated, which may be dona upon a sand-bath. The evapo* 

 ration is pushed to dryness, and thi salts that remain, having been 

 previously weighed, are thrown into a small platinum capsule, in 

 which they are heated to a dull red by means of a spirit-lamp, in 

 order to burn out the organic salts, and thus distinguish, by means 

 of a subsequent weighing, between them and the inorganic salts. 



The sand may be silicious or calcareous. The presence of car- 

 bonate of lime is readily ascertained by treating it with an acid which 

 will form a soluble salt with lime, such as hydrochloric, nitric, or 

 acetic acid. Effervescence shows the presence of a carbonate ; the 

 quantity of which may be estimated by weighing the sand dry before 

 and after its treatment with the acid, particular care being of course 

 taken to wash the remaining sand well before setting it to dry. 

 This, however, is an operation of little use, the great object is to as- 

 certain the quantity of sandy matter. Had we a particular interest 

 in ascertaining the presence and estimating the quantity of the earthy 

 carbonates contained in a sample of soil, it would be advisable to 

 make a special inquiry, inasmuch as the finely divided calcareous 

 earth being carried off along with the clay in the course of the wash- 

 ing, the sand obtained never contains the whole of the carbonate of 

 lime. 



The argillaceous matter procured by the washing is far from being 

 pure clay : it contains a quantity of extremely fine sand, particles 

 of calcareous earth, and if the soil contain humus, the more delicate 

 particles of this substance will also be included. 



To determine the quantity of humus, recourse is generally had to 

 its destruction by heat. A known weight of dried earth is heated 

 to redness in a capsule, and constantly stirred for a time, and when 

 no more of those brilliant points or sparks, which are indications of 

 the combustion of carbon, are observed, it is set to cool and then 

 weighed. This is the method which has been generally followed by 

 Davy and others. It would be difficult to find a method more con- 

 venient than this, but it is unfortunntely very inaccurate. Soils 

 dried at a temperature at which organic matter, such as humus, &c., 

 begins to change, still retain a considerable quantity of water in union 

 with the clay. This water is disengaged at the red heat required 

 for the combustion of the organic matters ; and as their quantity is 

 estimated by the loss of weight on the subsequent weighing, it is ob- 

 vious that the loss from the dissipation of water is added to that 

 which proceeds from the destruction of the humus. It is undoubted- 

 ly to this cause of error that we must ascribe the large proportions 

 of humus mentioned in the soils examined by Thaer and Einhoff ; it 

 is therefore better to restrict the examination to the determination 

 of the presence or absence of humus than to attempt to ascertain its 

 quantity by so imperfect a method. 



Priestley and Arthur Young were already aware that a more deli- 

 cate operation was required to determine the quantity of humus. 

 They recommend calcination of the soil in a close vessel, and that 

 the gaseous products should be collected. This mode of proceeding, 

 however, would have but slight advantages over that which I hava 



