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precision only by a systematic cheiTiical analysis, but their exist- 

 ence can be made to appear by the following simple tests. Put a 

 small portion of the marl in a flask or other thin glass vessel; 

 pour upon it some pure water, and heat it moderately. After 

 causing the water to dissolve in this way as much as possible, 

 remove the heat, and let it settle ; then decant the clear fluid into 

 some glass vessel, such as a wine-glass. If there is any copperas 

 present, it will be evident upon adding to the fluid a little lime- 

 water, which will produce a milky turbidness that after a little 

 while will become stained of a yellowish-brown colour. The 

 milkiness is owing to the formation of gypsum, and the brown 

 colour to oxide of iron from the copperas. Or in lieu of this, add 

 a solution of oak bark ; and if copperas be present, we shall have 

 a dark inky colour at once produced. 



A good marl will, upon being squeezed in the hand, fall asunder 

 again rather than bake into a tough doughy mass; and upon 

 being left in heaps to dry, will assume a light grayish green co- 

 lour and be extremely crumbly. It seems to be a very general 

 characteristic of the better class of marls, that they throw out a 

 white efflorescence or crust upon those grains which are most 

 exposed to the air: hence the very light colour externally which 

 some heaps of marl possess. This crust I have already shown 

 to consist usually of the sulphate of lime (gypsum), sulphate of 

 magnesia, and carbonate of lime. A drop or two of strong 

 vinegar, or any strong acid, will produce an effervescence or 

 frothing if it be the carbonate of lime; and should nothing of this 

 kind take place, we may set it down to be gypsum. Of course, 

 from the minuteness of the quantity of the white coating, much 

 care and accuracy of observation are demanded in doing this, in 

 order to avoid erroneous conclusions. 



Marls deemed equally good with the kind showing the efflo- 

 rescence, very frequently occur, exhibiting none of the white in- 

 crustation. 



It does not seem that any general rule can be given for distin- 

 guishing the fertilizing properties of a marl by its mere colour, as 

 it must appear from what has been said, that the peculiar shade 

 of colour is frequently owing to the colour of the intermingled 

 clay. When the mass, however, is comparatively free from clay 

 or common sand, and consists of little else than the greensand, 



