14S 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[April, 



tion, and yet can not be ilelcclccl by the thermometer ; deiirive It of it, and it 

 is again liquid. By absorbing its vapour as quicl<ly as formed under an air 

 pump receiver, water may be frozen, owing to llic abstraction of beat by i(s 

 own evaporation. It is from tliis cause that a wet soil is always a cold one, 

 for when the sun does shine on it, its warming influence is neutralized by the 

 action of evaporation. 



Tliese are tlie effects on plants from mere change of form. But as water is 

 decomposed largely by plants, its composition must be next considered. In 

 100 parts, water consists by weight of 



O-tygen 88-9 „ 8 



Hydrogen HI = l 



100- 1 



or, by measure, two of hydrogen to one of oxygen ; consequently oxygen is 

 16 times heavier than hydrogen, which is the lightest of gases, being about 

 l-15th of the weight of air. Kesins, with which some trees so abound, have 

 an excess of hydrogen in them. Now this can only come from the water 

 which they decompose, and consequently we .(ind that plants are continually 

 liberating oxygen, no doubt principally from the water of which they have 

 appropriated the hydrogen. 



The best method of ascertaining the composition of water is to decompose 

 i ; and if for that purpose electricity is employed, the advantage is gained of 

 being able to collect the products separately. In analysing the air, only one 

 of its components could be obtained, and that only by removing the other. 

 But by bringing the poles of a galvanic battery into water, the electric current 

 causes the hydrogen to separate from the oxygen, and each is evolved at a 

 dilferent pole, all the hydrogen at the negative, and all the oxygen at the 

 positive pole. By placing glass vessels over the poles the gases may be col- 

 lected, and it will then be found that the liydrogen will occupy twice the bulk 

 of the oxygen, but if it be weighed, it will be found to be only | of its 

 weight. A lighted taper immersed in tlie hydrogen is itself extinguished but 

 ignites the gas, which burns, if in contact with air, with a pale blue flame ; 

 immersed in the oxygen, its combustion is much more rapid and brilliant. 

 These gases, when mixed and ignited, explode with great noise, and re-form 

 water. But the best method of efl'ecting this is to screw on to the top of a 

 bell glass filled with the mixed gases, a Cavendish's apparatus, which consists 

 of a strong glass vessel, through each side of the neck of which passes a 

 platinum wire, which wires nearly meet in the interior ; having previously ex- 

 hausted this of its air, let the gases enter into it; then, after closing the stop- 

 cock, without removing the apparatus, send an electric spark through its 

 platinum wires from a Leyden jar, which will cause the gases to unite, and 

 the water formed will trickle down the sides of the Cavendish ; by again 

 opening the communication, more of the gases will rush into the partial 

 vacuum that is formeil, and it may thus be repeated tdl all the gases are 

 combined. It will thus be found that nine ounces of water can be decomposed 

 into eight ounces of oxygen and one ounce of hydrogen gases, which may 

 then be reconverted into nine ounces of water. Thus electricity is employed 

 to decompose and to recompnsc water. 



Next with respect to the carbonic acid. The base of this, carbon or char- 

 coal, has been very highly recommended for its good effects when applied as a 

 tup-dressing. This was once attributed to the j.lants feeding on it in some way 

 or other, but that idea is now discarded. It appears as though its action was 

 principally due to its absortion of ammonia. Many substances possess this 

 property of absorbing moisture and gases, but none to so surprising an extent 

 as carbon. If a piece of fresh-burnt charcoal be exposed to the air, it will 

 shortly be found to have increased considerably in weight, owing principally 

 to the water it has abstracted from the air. But this property varies according 

 to the source of the charcoal, as the following table exhibits :— 

 After one week's exposure of 



Charcoal from Lignum Vilse, it had gained 96 per cent. 

 !'"''■ ,. 13- 



» Beech „ 163 ,, 



•• Oak „ 16-5 



Mahogany ,, 18- „ 



But this properly of charcoal is shown more remarkably with respect to 

 the gases, many of which are rapidly absorbed to the extent of many times 

 the bulk ot the charcoal :— 



1 volume of charcoal absorbs of 

 Ammonia . . 90 volumes Carbonic oxide . 



Hydrochloric acid . 85 „ Oxyen 



Sulphurous . .65 „ Nitrogen '. 



Sulphuretted hydrogen 55 „ Hydrogen . 



Carbonic acid . .35 



On this account charcoal is frequently employed to sweeten putrid water, 

 by filtering it through it, and it is, after this operation, Ijetter fitted as manure 

 than previously, as the substances it withdraws from tlie water are beneficial 

 to vegetation. In this manner peat waters might be filtered, and the char- 

 coal employed as manure. 



9'42 voluues 



9-2.5 



7-5 „ 



1-75 „ 



But no jiroperty of carbon is so important to vegetation as its affinity for 

 oxygen, being by that means converted into gas, and so brought to the plant 

 in a fit state for food. In this manner it has derived its carbon, which in 

 many plants forms nearly one half of their weight. Not that carbon, when 

 pure and iincumbincd, shows any tendency, at ordinary temperatures, tocom- 

 bine with oxygen, but when in combination in the animal frame, or in the 

 vegetable tissue, by respiration and decomposition, then carbon is readily con- 

 verted into the gaseous acid. This being wafted over the surface of the leaf 

 is by it, with the assistance of solar light, decomposed, its oxygen being again 

 set free, its carbon, uniting with the elements of water, sometimes forming 

 woody fibre, sometimes gum, with an excess of hydrogen, forming resinous 

 substances, with excess of oxygen, forming acids. 



Lecture VII. 



When carbon In any state is heated to redness, it entirely passes ofT as 

 carbonic acid gas ; and if this be performed in a jar of oxygen instead of air, 

 the combustion is rapid, and the whole of the oxygen is converted into car- 

 bonic aciil. If the properties of this gas be then examined, it will be found to 

 extinguish alighted taper, and if an animal be inserted in it, it will be deprived 

 of life. Air containing one tenth of it causes drowsiness in man, folloHcd 

 by torpor and death; its action is then like that of a narcotic ; but when 

 pure, it suffocates instantly, causing spasm of the glottis. The oxygen is 

 combined with the carbon in this gas with so great an affinity, that it almost 

 baffles the chemist's art to separate them. He is obliged to have recourse 

 to his most energetic means in order to do that which the minutest plant is 

 doing so constantly, so easily, and so largely. Phosphorus has not the 

 power of abstracting the o.xygen, but if burning potassium be inserted into 

 this gas, decomposition is effected, potash formed, and carbon liberated, 

 which may be rendered evident by diffusing it through water. Carbonic acid 

 is heavier than air, in the proportion of 15 to 10, owing to which it may be 

 poured from vessel to vessel like water, this being rendered evident by the 

 extinction of a taper. It may even be transferred by letting down a little 

 bucket into a jar of the gas. But after a time it will be found to have 

 vanished, for it possesses a property common to all the gases, heavy or 

 light, of mixing with each other till they are perfectly blended. Also heat, 

 by expanding it, causes it to ascend, and it may thus be expelled from a 

 vessel by inserting a red hot ball. Owing to these properties, it is, that it 

 gets diffused in the air so evenly, and its injurious accumulation prevented. 

 Many substances absorb carbonic acid, as is proved by dipping a sponge in 

 caustic alkalies and placing it in a jar of the gas over water. In a short 

 time the water will rise and fill the vessel. Lime, also, rapidly absorbs it, 

 losing thereby Us causticity, being converted into chalk. From this chalk it 

 may be again driven by the action of a stronger acid, and this is the readiest 

 means of producing it. The difference of action of this gas and oxygen on 

 a combustible body may be beautifully illustrated by inserting a taper with a 

 long wick, just blown out, into a tall glass jar, the upper part of which con- 

 tains o,xygen, the lower carbonic acid ; the glowing wick is ignited on first 

 insertion, on lowering it to the region of the carbonic acid it is extinguished, 

 raising it up to the oxygen it is relit, and with care this may be repeated 

 many times. 



Water at the common temperature and pressure absorbs its own bulk of car- 

 bonic acid, but will dissolve more exactly in proportion to the pressure given. 

 It is in this manner that water is so strongly impregnated with it to produce 

 soda water, it communicating to liquids a peculiar agreeable pungency. Its 

 solution reddens vegetable blues, proving its acid quality. Plants can de- 

 compose it as readily when it is dissolved In water, as when in air, the action 

 taking place at the surface of the leaf. It is the peculiar characteristic of 

 animals to produce carbonic acid, of plants to absorb and decompose it. The 

 enormous increase of carbon in plants in a short time, is truly surprising, 

 and many experiments have been performed, the precautions taken being such 

 that the i)lant could derive carbon from no other source than the carbonic 

 acid of the air. Grown in calcined clay and water, seed peas have been found 

 to increase in three months as follows :— 



