1844.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL 



193 



AGRICULTURAL CHEMISTRY. 



By Professor Brands, F.R.S., &ic. 



Lecture VIII. — Delivered at the Royal Institution, March 10, 1844. 



(Specially reported/or this Journal,) 



Quitting now the consideration of the ultimate elements of plants, attention 

 will be directed to the substances elaborated from them by the plant, into 

 which, by its vital functions, it converts the carbon, hydrogen, oxygen and 

 nitrogen, by grouping them together in various proportions. This operation, 

 which is the peculiar province of the plant, and which the animal is com- 

 pletely unable to perform, is an operation almost as yet beyond the skill of 

 the chemist, who can in some cases, change one of these elaborated substances 

 into another, but cannot, except in some small isolated cases, construct them 

 from their elements. It has been shown iliat plants do not feed on carbon, 

 hydrogen, oxygen, or nitrogen in their crude state, nor until Ihey have com- 

 binfd into forms which may be considered as intermediate between organic 

 and inorganic matter, viz., water, ammonia, and carbonic acid, and decom- 

 posing and recombining their ultimate elements, produce these secondary 

 products, which are termed the proximate elements of plants, and are known 

 by the names of gum, starth, sugar, gluten, fibrin, albumen, lignin, resin, oil, 

 &c. These are of the greatest importance to the agriculturist, because it is 

 on these that animals live, and therefore on their relative proportion depends 

 the value of a crop. The proportion in which two of these pro.Nimate elements 

 aie present in some of the commonest crops, is shown by the following list : 



The proportion varies according to mode of growth, soil, climate, &c. The 

 manner in which the plant takes up the carbonic acid, the water and thj 

 ammonia, by the roots and leaves, regulated by light and heat, will be con- 

 sidered at the next meeting. The proximate elements thus produced, having 

 contributed to the support of one generation of animals, by decomposition 

 become the substances suitable for the food of plants, which are again de. 

 voured by the animal, and in this manner the chain of creation is kept up. 



In the ultimate analysis of the organic constituents of plants, we arrive bu' 

 at one result, the oft-told tale of carbon, oxygen, hydrogen and nitrogen ; 

 but in the proximate analysis we arrive at an infinite variety of substances' 

 in which the fecundity of nature, in producing from four elements so great a 

 diversity of distinct bodies, is powerfully shown. By burning and other 

 destructive means the ultimate elements are separated ; but to procure thg 

 proximate elements, much milder processes are adopted. As an illustration 

 of the ordinary methods adopted, the examination of the composition of wheat 

 flour may be taken. If this be put in a muslin bag, and agitated in water, it 

 will be seen that a white powder, which is the starch, will separate from it and 

 fall to the bottom of the water, whilst a peculiar adhesive fibrous substance 

 will be left in the bag, resembling birdlime. This, which is termed gluten 

 and on the relative quantity of which in flour its value as food depends, in- 

 stead of being a simple substance, is a very compound one ; for hoi alcoho 

 digested on it, takes up some substances and leaves fibrin, as it cools it de- 

 posits caseum, and if evaporated, will leave glutine. Ether digested on this, 

 dissolves from it some fat. The water in which the flour was washed holds 

 gum and sugar in solution, and after filtration from the starch, on boiling, a 

 substance is separated analiigous to white of egg, termed albumen. In this 

 manner can be separated, from an apparently homogeneous substance, the 

 following proximate elements ;— gluten, fibrin, caseum, glutine, fat, starch 

 sugar, gum, and albumen. In this manner are the processes conducted for 

 separating organic substances into their several proximate parts. These 

 may be grouped under four heads, according to their ultimate elements. 



Class I. Where the carbon has hydrogen and oxygen in the exact propor- 

 tion for forming water. 



Class II. Where the oxygen is in excess over this proportion. 



Class III. M'here the hydrogen is in excess. 



Class IV. Where, in addition to the carbon, hydrogen, and oxygen, nitro- 

 gen forms a part. 



The first class, which may be considered as composed of carbon and water 

 contains gum, sugar, starch, wood, and other neutral bodies. 



The second class, in which there is excess of hydrogen, contains the resinous 

 bodies, such as resin, wax, oil, fat, &c. 



The third class, with excess of oxygen, contains the vegetable acids. 



The fourth class, characterized by the presence of nitrogen, contains the 

 alcaloids, gluten, albumen, gelatine, 8ic. 



The peculiarities of each group must be next considered. The first giou^j 



can be converted the one into the other slightly by art, but readily by nature. 

 And when tlie similarity of their chemical composition is con8i<lered, their 

 easy convertibility is not surprising, though their physical properties appear 

 so essentially different. Their composition is given below :— 



Sugar is characterized by its solubility in water and alcohol, by its sweet 

 taste, by being soluble in alkalies, and by its crystallizing readily. There are 

 several varieties of sugar, but the two principal are cane and grape sugars, 

 the latter being that which gives the sweetness to fruits, as to grapes, plums, 

 pears, apples, figs, &c. Although the difference between cane and grape 

 sugar in composition is merely that the latter contains one atom more water, 

 yet the former possesses sweetening properties in a much more eminent de- 

 gree. Unfortunately for domestic economy the sweeter sugar can be artifi- 

 cially converted into the one less sweet, but not vice versa. These sugars 

 can be distinguished from each other by the action of acids ^il alkalies. 

 Sulphuric acid completely decomposes cane sugar, leaving nothing but 

 a carbonaceous mass, whilst it but slightly colours grape sugar. Honey, 

 which consists principally of grape sugar, is merely rendered into a fluid like 

 treacle, whilst a solution of sugar candy is acted on very strongly, little else 

 but a mass of charcoal being left. Alkalies have a contrary effect, acting 

 strongly upon grape sugar, and but slightly on cane. There are several otlicr 

 means of distinguishing between various sugars, and amongst the best is their 

 action on sulphite of cupper. 



Gum, which belongs to this group, is soluble in water and insoluble in 

 alcohol, and hence comes a ready method of separating sugar and gum when 

 dissolved in water; it is only necessary to add alcohol, and the gum is pre- 

 cipitated. This method is frequently resorted to in analysing substances for 

 food. There are several modifications of gum, differing from each other in 

 some of their properties, and as types of three kinds may be taken, gum 

 Iragacanth, the gum which exudes from the cherry and plum trees, and gum 

 arable. Some of these gums can be converted into other kinds with great 

 facility. Immersed in boiling water, gum tragacanth swells considerably ; 

 boiled for a long time, it passes, through cherry-tree gum, into gum arable. 



Woody fibre is insoluble in nearly all menstrua. When a piece of wood 

 has been subjected to the action of water, dilute acids and alkalies, everything 

 soluble is dissolved out, and a fine white powder is left, which is the woody 

 fibre or lignin, and is that part of llie vegetable which is worked up into 

 paper. There are several varieties or modifications of this, forming both 

 elder pith and the hardest wood. 



Starch, unlike the other proximate elements of plants, has an organized 

 structure. When examined by a microscope before removal from the plant 

 it is seen in the cells of the wood growing like a fruit or berry, liaving a spot 

 on the granule supposed to be a point of attachment. It is composed of eon- 

 centric layers, with a fluid in the interior soluble in cold water, but wliich is 

 protected by the exterior, which is not soluble till immersed in hot water, 

 when, bursting, the whole forms the well known gelatinous mass. A beauti- 

 ful and delicate test is used for starch, viz , a solution of iodine. Several 

 varieties of starch are known, amongst which the best known are potatoe 

 starch and wheat starch. One lately introduced, called tons les mois, con- 

 sists of very large grains. Starch is very abundant in most vegetable sub- 

 stances used for food, and is essential for nutrition. To ascertain the 

 presence of starch, pour on the substance a solution of iodine in water, and a 

 beautiful blue iodide of starch is formed if it is present. A cut potatoe or a 

 slice of bread shows this exceedingly well. 



The transmutation of this class of substances takes place very readily. 

 Starch or wood are easily converted into gum or sugar. Cotton or sawdust, 

 stirred up with a little sulphuric acid, is reduced to a gummy mass ; this, 

 boiled for 24 hours, is converted into sugar, whicli may be separated from the 

 acid by the addition of chalk, and filtering the sweet liquid from the sulphate 

 of lime thus formed. It may then be crystallized. But the sugar thus ob- 

 tained is grape sugar, which has but little sweetening power. Cane sugar 

 boiled with acid is in a similar manner converted into grape sugar. Starch 

 is very easily transmuted. If it lie carefully heated to a point short of char- 

 ring, it is converted into a kind of gum, known as dextrine. It has then lost 

 the property of forming a blue with iodine, and is readily soluble in cold 

 water. It is manufactured largely in France, and is used there to stiffen 

 ribbuns. It is also the principal ingredient in the cement at the back of the 

 postage stamps, as it is very adliesive. 



