1839] 



FARMERS' REGISTER 



427 



stances, when placed in a warm damp atmosphere, 

 undergo a series of chanijes, which result in their 

 entire dissolution. When thus decomposed, they 

 are not, however, separated into their ultimate ele- 

 ments, viz., carbon, oxygen, Iiydrogen and nitro- 

 gen, but into certain simple binary compounds of 

 these elements, among the most abundant of which 

 are, carbonic acid (c + 2 ox.,) water (h -\- ox.) 

 hght carburetled hydrogen (c -[- 2 h,) and ammo- 

 nia (3 h -f- n). Those which are not thus decom- 

 posable, will all undergo decomposition at a tem- 

 perature less than that ofa white heat. They are 

 also frequently convertible info each other ; for 

 instance, starch, which is isomeric with sugar, is 

 readily convertible into that substance, and does 

 in fact undergo such a change during the germi- 

 nation of the seed; this is no more than we should 

 expect fiom our knowledge of their composi- 

 tion. 



3rd. In general, vegetable substances cannot 

 be formed by art. Although we are perfectlj'^ well 

 acquainted with the composition of sugar, for in- 

 stance ; we know that it is composed of" 6 equiva- 

 lents of carbon, 6 of hydrogen, and 6 of oxygen, 



and we can easily procure such of these substan- 



cesj and present them to each other in just the pro- 

 per proportions to form sugar, yet in no way which 



has yel been discovered can we cause them so to 



unite as to form that compound. That this should 



be the case with those substances which possess a 



distinct organization, is not a matter of surprise, 



but that ii should likewise be the case with those 



which possess no other than a crystalline struc- 

 ture, is probably to be attributed to the imperfec- 

 tion of our knowledge of vegetable chemistry. 



We cannot indulge a reasonable hope of ever 



being able, by any process of art, to form cotton 



from simple inor<ianic carbon and water, although 



we know it to be composed principally of those 



materials ; but we may indulge a hope that, 



when our knowledge of chemistry is more com- 

 plete, we may discover a way of forming sugar 



from carbon, oxygen and hydrogen, either by the 



direct union of these elements, or else of some of 



their binary compounds; at least, there is nothing 



absurd in the indulgence of such a hope. But 



whatever may be the fact in lime to come, it is 



certain that we know of no process by which they 



can be formed at the present day ; and this should 



be borne in mind in examining into the elaboration 



of the sap, as we must not expect to find that it is 



effected by any process which has an exact paral- 

 lel in the arts. 



We are now prepared to discuss the question, 



by what means are plants enabled to effect the 



elaboration of their sap. As that sap enters the 



root, it is a thin fluid consisting principally ofwater 



containing carbonic acid gas in solution ; from 



this raw material, in some way or another, plants 



are enabled to form all the components of the ve- 

 getable structure, the wood, the starch, th^ sugar, 



the gum, the oil, the resin, the a'nds, &c. The 



question is, by what means is it that plants are 



enabled to effect these changes. To give a full question lias gained a proportionable quantity of 



those chemical changes which constitute what is 

 termed the elaboration of the sap. arc effected 

 through the agency of solar light, or if not by its 

 direct agency, it is necessary to enable the plant 

 to effect them. How it is that light is able to 

 change the chemical relations of substances, so as 

 to cause those to combine, which would otherwise 

 have remained separate ; or to cause those to se- 

 parate which would otiiervvise have remained 

 combined, is a tnalter of whicli no satisfactory ex- 

 planation has ever as yet been given; and yet there 

 is no fact in chemistry which is better established 

 than ihaf it does act in this way. If a bottle 

 partly filled with nitric acid, be set away in a dark 

 place, it will undergo no change; but if it be expo- 

 sed to direct sun-light, a decomposition ofa portion 

 of the acid at once commences, and the whole of 

 the upper part of the bottle vvill soon become filled 

 with a mixture of oxygen and nitrons acid. If a 

 mixture of equal volumes of chlorine and hydrogen 

 be enclosed in a bottle, and light be carefully ex- 

 cluded, no combination will take place, it matters 

 not how long they be thus kept mixed ; but if the 

 mixture be exposed to diffuse day-light, the two 

 gases will gradually combine, and form hydro- 

 chloric or muriatic acid; if the mixture be exposed 

 to direct sunlight, the combination takes place as 

 instantaneously, and with as violent an explosion, 

 as if caused bv the contact of flame or by the elec- 

 tric spark. These facts, and numerous others of 

 a similar character, which might be mentioned 

 did it seem necessary, establish beyond question, 

 the principle that light does possess the power of 

 so altering the ordinary chemical relations of bo- 

 dies as to cause combinations and decompositions 

 which otherwise would not lake place. 



The principal agency of liglit appears to be ex- 

 erted in effecting the decom.position of carbonic 

 acid, and in fixing the carbon thus obtained, in the 

 tissues of the plant. Some ofthe evidence on which 

 this statement rests, has been given in a preceding 

 chapter ; I will here add only a few remarks of 

 De Candolle, in which he has embodied the 

 results of numerous experiments performed by 

 himself. "If" says he "two plants are ex- 

 posed, one to darkness, and the other to the sun, 

 in close vessels and in an atmosphere containing a 

 known quantity of carbonic acid, and are removed 

 at the end of twelve hours, we will find that the 

 first has diminished neither the quanlit}' of oxygen 

 nor of carbonic acid; and that in the second, on the 

 contrary, the (juantity of carbonic aci.i has dimin- 

 ished, while the quantiiy of free oxygen has 

 increased in the same proportion. Or if we place 

 two similar plants in close vessels in the sun, the 

 one in a vessel containing no carbonic acid, and 

 the other in air which contains a known quantity 

 of if, we shall find tlr.ir the air in the first vessel 

 has undergone no change, while that in the second 

 will indicate an increase of oxygen proportioned to 

 the quantity of carUinic a'-id which lias disap- 

 peared ; and if the ex|)eriment is conducied with 

 sufficient care, we shall discover that the plant in 



and complete answer to this question, is impossible 

 in the present state of our knowledge of vegetable 

 physiology; yet there are some facts which have 

 been ascertained by experiments carefully perform- 

 ed and often repeated, which it may be important 

 to the practical agriculturist to know. 



The most important of these is, that many of 



carbon. Therefore we conclude that the carbonic 

 acid which has disn[ipeared, has given its oxygen 

 to the air, and its carbon to the plant, and that 

 this eflect has been prodnced solely by the action 

 of solar light." All the experiments which have 

 hitherto been made, lead us to the same conclusion 

 that solar light is absolutely necessary to plants, to 



