128 



THE POPULAR EDUCATOR. 



will assume the charcoal employed to be absolutely pure. We 

 burn, then, an absolutely pure bit of charcoal in atmospheric air, 

 and it totally disappears ; nothing remains : not the smallest trace 

 of ashes ; all is gone. What, then, has become of the charcoal ? 

 This is not a chemical book, therefore we have not space to go 

 into the matter in all its chemical relations. We must, there- 

 fore, content ourselves by saying that the charcoal, by burning, 

 is converted into a gas termed the carbonic acid gas. This 

 carbonic acid gas is quite invisible, therefore one might look for 

 it in vain ; but it has a smell and a taste, therefore we might be 

 conscious of its existence, even though we had no means of 

 catching it. But we have such means. If this gas comes in 

 contact with lime, or potash, or soda, either of these substances 

 lays hold of it, combines with it, or, if we may be pardoned the 

 expression, licks it up. Therefore, by setting a little quicklime 

 in places where carbonic acid gas exists, we may catch it just as 

 readily as we can catch a mouse in a trap ay, more readily, 

 because a mouse may at least choose whether he go into the 

 trap or stay out of it ; but the carbonic acid gas has no such 

 choice ; if it comes in contact with the trap of lime, in it must go 

 without fail. Now, what we want 

 to come at is this. Although a 

 a piece of charcoal when burnt goes 

 away in an invisible form, it never- 

 theless only makes a new acquaint- 

 ance and puts on a mask. We can 

 catch it, can unmask it, and get the 

 charcoal out of it once more. 



Carbonic acid gas is a poison, as, 

 we dare say, most of our readers 

 know ; hence the danger of sitting 

 near a pan of burning charcoal. 



Proceeding with our chemical re- 

 marks, we must now go on to say 

 that combustion is far from being 

 the only source of carbonic acid gas: 

 thus it is given off during fermen- 

 tation, is given off from effervescent 

 wines, such as champagne and 

 sparkling moselle, is given off from 

 ginger beer and soda water, and, 

 what is far more to our purpose, is 

 given off from the lungs of animals 

 by the act of respiration. Indeed, 

 -bhe functions of animal digestion 

 and respiration taken together may 

 be considered as a sort of combus- 

 tion, and are actually termed com- 

 bustion by some authors. The simi- 

 larity is indeed striking, as a little 

 contemplation will serve to demon- 

 strate. Thus, if we throw a lump 

 of coal into a fire-place, heat is 

 given out, and gaseous matter 

 (chiefly carbonic acid) escapes. If we swallow a morsel of food, 

 it is digested, heat is given out, and carbonic acid escapes. In 

 the former case carbonic acid escapes by the chimney, in the 

 latter case by the lungs. One chemical point yet remains to be 

 explained before the student will be in a position to understand 

 the functions of a vegetable leaf. The carbonic acid, of which 

 -we have been speaking, is a gaseous compound of charcoal, 

 termed by chemists carbon and something ; that something is 

 oxygen, the vital principle of the air. Now, the bulk of 

 vegetable bodies is made up of carbon, otherwise how could we 

 get charcoal in the ordinary way ? And this bulk, this carbon, 

 is got out of the air. Yes, the largest tree, whatever its size, is 

 for the most part formed of carbon, and all this carbon once 

 existed in the gaseous form. Philosophers have made calcu- 

 lations, from which it appears that the total amount of carbonic 

 acid thus floating about in the atmosphere amounts to the 

 enormous quantity of many tons, and that tons of carbonic 

 acid hover over each acre of ground, ready to give up its carbon 

 to vegetables which require this substance. Before quitting 

 this subject, we must not forget to direct the reader's attention 

 to the beautiful provision by means of which the amount of 

 carbon necessary to be got rid of from the animal economy is 

 evolved in the particular form of gas. Even supposing no 

 Positive injury to result, yet just think how dirty and begrimed 



THE ASH. 



we should be if we were always puffing out charcoal dust with 

 every expiration ! We do not expire a small quantity either, no 

 less than thirteen ounces of charcoal being evolved during 

 twenty-four hours from each human individual. Had not some 

 provision been adopted for enabling carbon to be thus evolved 

 in a gaseous form, we should all have been blacker than 

 chimney-sweeps. What a miserable state of things would this 

 have been ! 



Respiration, then, is the chief function of leaves, but it is not 

 the only function ; they also serve as evaporative organs, by 

 means of which the plant gets rid of excessive moisture j and in 

 this respect, again, they present a striking analogy to animal 

 lungs. Who amongst us is not aware that our breath contains 

 moisture ? 



SECTION VII. ON THE FOEM AND MODIFICATIONS OF 

 LEAVES. 



Having described the general functions of leaves, we must now 

 proceed to examine their forms, and to learn the terms by which 

 those forms are designated, otherwise we should not be able to 

 describe a plant in such a manner 

 that a person would understand 

 our description. As in many othei- 

 parts of Botany, the student wilJ 

 here encounter some long names ; 

 they are very useful names, never- 

 theless, and require to bo under- 

 stood. 



In the first place, taking a general 

 review of the aspect of leaves, it 

 will be evident to the reader that 

 their form is exceedingly varied, 

 as is also their manner of attach- 

 ment to the stem, to say nothing of 

 such characteristics as softness, 

 hardness, thickness, thinness, and 

 so forth. As regards their attach- 

 ment to the vegetable, some leaves 

 grow directly out of the stem, or, 

 in figurative language, may be said 

 to sit upon the stem. Such leaves 

 are termed by botanists sessile, from 

 the Latin word sessum, a part of the 

 verb sedeo, to sit. Others are at- 

 tached to the parent stem by a little 

 stem of their own. Now, this leaf- 

 stem, or foot-stalk of a leaf, bota- 

 nists denominate a petiole, from the 

 Latin petiolus, a little foot, and 

 leaves thus supplied with a petiole 

 are said to be petiolate. Again, 

 some leaves are attached to the 

 parent stem exactly opposite each 

 other, consequently they are said 

 from this circumstance to be opposite or opposed. Others are 

 alternately attached, from which circumstance the denomination 

 alternate is given to them. All these characteristics are very 

 important, not only in enabling a botanist to describe the con- 

 figuration of plants in the fewest possible words, but in 

 enabling him at the same time to separate plants into natural 

 groups and alliances. 



Again, some leaves are single in themselves, as is the case 

 with those of the apple-tree; whilst others are made up of 

 several little leaflets, as we see, for example, in the ash. Hence 

 arises the very natural distinction of leaves into simple and 

 compound. 



The forms which leaves assume are so very numerous, that 

 botanists are accustomed to indicate them by the similarities 

 which they manifest to natural objects. Some are like shields, 

 for which reason they are termed peltiform (Latin, pelta, a 

 shield) ; others are like hearts, whence they are termed cordiform 

 or cordate (Latin, cor, conlis, a heart). Some resemble feathers, 

 others are jagged like a saw, whence arise the denominations 

 penniform (Latin, penna, a feather or wing), serrate or serra- 

 tiform (Latin, serra, a saw), and so forth ; but we shall give in 

 our next lesson dra ings of the chief varieties of leaves, from an 

 inspection of which the various names respectively applied to 

 them will be rendered more evident. 



