56 MANURES. 



When there is a yellowish or whitish incrustation upon the marl in a bank, 

 after the moist surface has remained for some time exposed to the weather, it 

 is indicative of the existence of a portion of copperas or alum-earth, the hurtful 

 nature of which has already been explained. 



An astringent inky taste will very often detect the presence of these noxious 

 substances at times when no such efflorescence shows itself. If the quantity 

 be too small to betray itself distinctly to the palate, and we are still in doubt as 

 to their presence, other more rigorous tests are within our reach, and as these 

 astringent matters are so unquestionably pernicious in their action, it must be 

 of importance to have the means of determining in what proportions they abound 

 in different marls. This can be effected with precision only by a systematic 

 chemical analysis, but their existence can be made to appear by the following 

 test. 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 dear fluid into some glass vessel, such as a wine-glass. If there has 

 been any copperas present, it will be proved by adding to the fluid a little lime- 

 water, which will produce a milky turbidness, that after a little while will be- 

 come 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 

 retain a light greyish-green colour 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 consists of the sulphate of lime (gypsum) and carbonate 

 of lime, but more usually of the former. A drop or two of strong vinegar or 

 any strong acid will produce an efflorescence or frothing if it be the carbonate 

 of lime, a~nd 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, 

 inuch care and nicety of observation are demanded in doing this, in order to 

 avoid erroneous conclusions. 



I do not state that marls equally good with that kind having the efflorescence 

 do not very frequently occur and exhibit none of this white incrustation. 



It does not seem that any general rule can at present be safely given for dis- 

 tinguishing the fertilizing properties of a marl by its colour. The truth of this 

 must appear from what has been said about the peculiar shade of colour being 

 so 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 single material, the green-sand, my observations go to establish 

 that the rather dark green variety is more potent in its effects upon the land 

 than the very light green which sometimes overlies it. 



The presence or absence of shells I look upon to be a point of but little mo- 

 ment, for I find that several of the most active marls in the region, show no 

 traces of fossils in them. The whole amount of carbonate of lime in the shape 

 of fossils, and in that of the occasional white incrustation upon the grains, can 

 in very few instances amount to one per cent., while as my analyses show that 

 the lime chemically combined with the other ingredients in the green grains, 

 is sometimes as mudi as ten per cent., and that the potash amounts almost to 

 fifteen per cent. 



There yet remains, however, the most important, and by far the most diffi- 

 cult inquiry, namely into the exact constitution of the green grains, in order 

 to determine the per centage of the several ingredients or, in other words, 

 the richness of the marl in potash and lime. I had entertained hopes, that the 

 external aspect of the grains might perhaps depend in part on the presence and 

 proportion of these bodies, and that mere inspection, after multiplied analyses 

 were made might enable any one with certain directions, to inform himself 

 whether a marl abounded in these essentials or not. But, I find that so far 

 from being a mineral of definite and constant proportions, as some inineraio- 



