1844.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



U 



AGRICULTURAL CHEMISTRY. 



Lecture II. Br Professor Branue, F.R.S. 



(Speciallij reported for this Journal.) 



The history of lime and its salts, which we commenced in the preceding 

 lecture, necessarily leads us to speak upon that important article, sulphate 

 of lime, or gypsum, which is a compound of sulphuric acid and lime ; and as 

 I have called your attention to the ultimate elements of lime, so must I do 

 with regard to sulphate of lime, which is to he regarded as containing the 

 metallic base of lime, united with sulphur and oxygen. Sulphuric acid is a 

 compound of sulphur and o.xygen. Lime is a compound of calcium and oxy- 

 gen ; therefore, the lUtimate components of sulphate of lime are sulphur, cal- 

 cium, and oxygen. 



Now I will proceed to build up this substance ; and for this purpose I will 

 first make sulphuric acid, which 1 do by burning sulphur in oxygen, as I 

 previously made carbonic acid by burning cai<JOn in oxygen. To burn the 

 sulphur in this way it must be mixed with a small portion of nitre. In the 

 atmosphere sulphur burns with a thin pale light, but when introduced into 

 oxygen, it burns with a beautiful purple light, and produces sulphuric acid- 

 If 1 take this substance, then, and add it to lime, I have sulphate of lime. 

 The same substance may be obtained by using the sulphuric acid at once 

 and lime, then you may see the immense affinity that exists between sulphu- 

 ric acid and lime, and how much heat is evolved during the combination. In 

 all cases their proportions are quite definite, and we have 40 parts of sulphu- 

 ric acid always combining with 28 parts of lime. 



Sulphate of lime is met with in a variety of forms, and is known under a 

 variety of names. There is a variety met with in this country called anhy- 

 drous sulphate of lime, which is sulphate of lime minus a certain quantity of 

 water. This substance is very difficult of solution in water, requiring about 

 350 parts of water to 1 for that purpose. It is often met with in the form of 

 crystals, and is frequently found in digging wells about London in the blue 

 clay ; it is also pretty abundant in the clay at Sholover Hill, near Oxford, 

 and elsewhere. It contains 08 parts of dry sulphate of lime, with about 18 

 parts of water. If a gentle heat be applied to this, the water goes oti', and the 

 substance crumbles down to what is well known as Plaster oi Paris. If it is 

 mixed with water it regains the water it had lost by the heat, and reproduces 

 a crystallized or solid compound. It deserves notice, however, that if you 

 overheat sulphate of lime it loses this property of re-combining with water, 

 and you thus may destroy its property as a plastic stone. 



Now you will find sulphate of lime diffused pretty generally through com- 

 mon spring water in the neighbourhood of London, and some of the shallow 

 wells of London are almost concentrated solutions of gypsum. Its presence 

 may be readily tested by oxalate of ammonia, which immediately renders 

 water turbid which contains it. It is this which renders water hard, and un- 

 fit for mixmg with soap. AVhen organic matter of any kind comes into con- 

 tact with sulphate of lime and water, there is a tendency to mutual decom- 

 position, whereby the sulphuric acid loses its oxygen and becomes converted 

 into sulphur, and then we have sulphuretted hydrogen very often evolved. It 

 is not uncommon to find rivers of Africa and Asia which contain great quan- 

 tities of sulphuretted hydrogen. 



I have called your atteniion to the existence of sulphur in sulphate of lime, 

 putting aside its secondary combination, sulphuric acid, because there are 

 certain crops which contain not only sulphate of lime, but sulphur, in its 

 peculiar and, comparatively speaking, tree stale, such as mustard, horse- 

 radish, and other plants having their flavour. 



The principal sources of sulphate of lime in this country are, first, the red 

 marl and salt deposits. It is found in great quantitiea in the salt-pits of 

 Cheshire ; it abounds also in the Alps and other primitive rocks; and it is 

 also met with in abundance in the tertiary strata in the neighbourhood of 

 Paris, from which circumstance it derives the name of plaster of Paris. 



With regard to the uses of sulphate of lime in agriculture, there can be 

 little doubt that owing to its action it may be called the lite of plants ; for it 

 we examine a plant, we shall find that it constitutes a considerable part of its 

 texture. The notion of its attracting and retaining moisture is incorrect, as it 

 does neither one nor the other, nor does it promote decomposition. It does, 

 hoH ever, assist the growth of certain crops, and if we examine the ashes of them 

 when burnt, we shall find sulphate of lime. It is found in the ashes of clover 

 and trelbil ; but not in any considerable quantity in those of wheat, barley, 

 oats, beans, or peas. And it is found that to these crops sulphate of lime is 

 of no use whatever, whereas it is a fertilizing manure to others. The fact is, 

 that plants which do contain it never grow well on soils that are destitute of 

 it, and this is found to be the case witfi other inorganic substances. Plants 

 which contain carbonate of lime will not grow in a soil that does not contain 

 it. There are many plants which contain salt, and they will not grow where 

 it is not. Wheat must have phosphate of lime, because wheat possesses it, 

 and so on. 



It is Stated upoa goot] authority tbat aa grdinary crop of clover rei^uires 



from about \i to 2 cwt. of sulphate of lime per acre. And this proportion is 

 recommended for use by those best versed in the subject. Of course it is of 

 no use where the soil already contains it ; and it is of importance in practi- 

 cal agriculture to ascertain what the soil does contain. If fields winch once 

 gave luxuriant crops of red clover no longer do so ; if the young plants spring 

 u|i soon, but die as the summei advances, it may be concluded, wiihout any 

 chemical analysis, that gypsum is wanted. There are other sources of gyp- 

 sum amongst manures which are known. Peat ashes contain 12 or II per 

 cent, of sulphate of lime, and it is also found in coal. 



There is another statement with regard to gypsum, which we do not find 

 borne out by experience— that is, its capabilily of fixing ammonia. We know 

 that it is highly essential to vegetation. Anything, therefore, which would 

 tend to fix it, and collect it, would be very valuable. Carbonate of ammonia 

 is produced in stables and other places where animal matter is undergoing 

 particular stages of decomposition, and though very injurious both toman 

 ami beast, it is a substance of great value as a manure. It has been said that 

 sulphate of lime will fix this substance, and that if a stable be strewed with 

 it, that purpose will be effected. But it does so very imperfectly. If I take 

 sulphate of lime and add to it a solution of carbonate of ammonia, I shall 

 find, after a time, that sulphate ot ammonia will be formed, and that car- 

 bonate of lime will be thrown down ; and it is stated that this change will 

 take place when stable manure is mixed with sulphate of lime. We can do 

 it in the laboratory, but it is a very doubtful process in the stable ; and we 

 should always take care that we do not apply particular experiments of the 

 laboratory to practical general cases— the reasoning may be good, but the 

 practice is bad. 



Anothersaltof lime on which we must say a few words is, phosphate of 

 lime. This substance occurs in considerable quantities in the bones of all ani- 

 mals, and it is quite clear that all the phosphate of lime in our bones must 

 come from the soil. Plants contain it j so do animals, and we are constantly 

 taking it in, in wheat, bread, &tc. It consists of phosphoric acid ami lime — 

 and now we come to the production of phos;ihoric acid. That extraordinary 

 substance phosphorus, wliich is a part of our bones and blo;id, and which is 

 an essential part of that most important organ, the brain, exists in the vege- 

 table world in the state of phosphoric acid. If we burn phosphorus in oxygen 

 we have a brilliant combustion, and phosphoric acid is the result, and to this 

 we have only to add lime to proiluce phosphate of lime, or wliat has been 

 called bone eartli. 28 parts of lime, 36 parts of phosphoric acid, and 18 of 

 water, are the components of 78 parts of crystallized phosphate of 1 me. 



How is the soil supplied with phosphate of lime? This is a question which 

 has lately very much engaged the attention of agricultural chemists. In the 

 first place, it derives it from artificial manures, from bones, and in bones you 

 have an extraordinary compound. Bone is composed of an earthy part and 

 an animal part. If we digest bone in acid we dissolve the earihy part and 

 have tlie animal part remaining ; and if we burn bone we destroy the animal 

 part and leave the earthy part. We find that this eartliy part of bone is in- 

 destructible, but the animal part undergoes decay ; and if we examine bones 

 that have been long exposed to the air, we find that they are comparatively 

 light, and that a great deal of the animal part is gone. The animal part of 

 bones may, however, be preserved lor a long time, which is proved, by 

 taking bones that have been long immured in caverns, where we find the 

 animal part quite perfect. Dr. Buckland, indeed, out of some antediluvian 

 bones made very good gravy ami soup. 



There is another very curious source of phosphate of lime, although it is 

 one which has been over-rated as regards its quantity. This is a substance 

 called the coprolite, which is the excrement of certain antediluvian animals who 

 were so voracious that they fed on each other. They consequently ate a 

 great quantity of bones, and their excrement is little more or less than plios- 

 phate of lime. This excrement does exist in some soils, but when Liebig tells 

 us that here we have stores of phosphate of lime that are to suffice for all 

 agricultural purposes, I think he goes on at rather too rapid a rate. Tlie 

 subject is, however, an important one, and any source of phosj hate of lime 

 that can be discovered is of great imfiortance. Another source is guano, 

 which is the excrement chiefly of birds, and it contains, amongst other 

 things, a large quantity of phosphaie of lime. 



Phosphate of lime, as a mineral product, has been discovered in various 

 places in this country. Some beautiful crystal specimens of it from Devon- 

 shire are on the table. It has been found also in Bohemia ; ami at I'lstrama- 

 dura, in Spain, where Dr. Daubeny discovered considerable quantifies of it 

 almost in a pure state. How far it is available as a manure, is a question 

 which I cannot go into now. It is also found in some marls, and in clialk, in 

 oyster shells, corals, ihe crust of metals, and in clay-slate ; or when I say 

 pliosiihate of lime is found, it is in reality phosphate of alumina, which after- 

 wards appears occasionally to form phosphate of lime. 



Such is the importance attached to phosphate of lime by Liebig, that he 

 considered it the most essen;ial ingredient in the soil with reference to parti- 

 cular crops, and more especially wheat. There can be no doubt that, to a 

 certain extent, this is the case, and a number of important questions arise as 

 to the best mode of obtaining and api'lying it. The finer the state of division 



