AGRICULTCRAL WJSEUM, .^73 



To find lis composition, pour a few ounces of weak 

 but; pure spirit of nitre or common salt into a Florence 

 ildsk ; place them in a scale and let them be balanced^ 

 tlicn reduce a few ounces of dry marl into powder, and 

 let this powder be carefully and gradually thrown into 

 the flask, until after repeated agitation no effervescence 

 is any langer perceived; let the remaind;'r of the powder- 

 ed marl be then wcig!ied, by which the quantity project- 

 ed will be known ; let tiie balance he then restored ; the 

 difference of weight between the quantity projected and 

 that requisite to restore the balance, will discover the 

 weight of air lost djrin:^ effervescence ; if the loss 

 amounts to 13 per cent, of the; quantity of marl proj,ect- 

 cd, or from 13 to 32 per cwt. the marl essajed is cal- 

 careous marl. Tuis experiment is decisive when we are 

 assured, by the external eliaraeters above mentioned, 

 tliat the substance employed is marl of any kind; other- 

 wise some sorts of sparry iron ore may be mistaken 

 lor marl. Tne experiments to discovei' the argillaceous, 

 ingredient (being too difiicult for iarmers) I omit. The 

 residue left after solution, bein;; well washed, will, 

 when duly heated, generally harden into a brick. 



ArglUaccoiis Marl contaisis from 68 to 80 per cent, of 

 clay, & consequently from 32 to 20 per cent, aerated calx. 

 Its colour is grey or brown, or reddish brown, or yellow- 

 ish or bluish gray. It feels more unctuous than the for- 

 mer and adheres to the tongue : its hardness generally 

 much greater. In water it falls to pieces more slowly, 

 often into square pieces : it also more slowly moulders 

 by exposure to the air and moisture if of a loose consis- 

 tence : Ife haidcns when heated, and forms an im})erfect 

 brick. It effervesces with spirit of nitre, or common 

 salt, but frequently refuses to do so with vinegar. When 

 dried and projected into spirit of nitre, in a Florence 

 flask, with the attentions above mentioned, it is found 

 to lose from eight to ten per cent, of its weight ; the un- 

 dissolved part, well washed, wiU, when duly heated; hap- 

 den int'j a brick. 



