Chapter 11

In the matter of the feeding of the two great living worlds we might perchance light upon some adequate grounds for making up the one kingdom from the other. What the consideration of form, movement, chemical composition, and microscopic structure could not effect for us in this way, it might be supposed the investigation of the diet of animals and plants would render clear. Our hopes of distinguishing the one group from the other by reference to the food on which animals and plants subsist are, however, dashed to the ground; and the diet question leaves us, therefore, when it has been discussed, in the same quandary as before.

Nevertheless, it is an interesting story, this of the nutrition of animals and plants. A large amount of scientific information is to be gleaned from such a study, which may very well be commenced by our having regard to the matters on which a _green_ plant feeds. I emphasize the word "green," because it so happens that when a plant has no chlorophyl (as green color is named in the plant world) its feeding is of diverse kind to that which a green plant exhibits. The mushroom or other fungus may be taken as an illustration of a plant which represents the non-green race, while every common plant, from a bit of grass to an oak tree, exemplifies the green-bearing order of the vegetable tribes.

Suppose we were to invite a green plant to dinner, the _menu_ would have to be very differently arranged from that which would satisfy a human or other animal guest. The soup would be represented for the plant's delectation by water, the fish by minerals, the joint by carbonic acid gas, and the dessert by ammonia. On these four items a green plant feeds, out of them it builds up its living frame. Note that its diet is of inorganic or non-living matter. It derives its sustenance from soil and air, yet out of these lifeless matters the green plant elaborates and manufactures its living matter, or protoplasm. It is a more wonderful organism than the animal, for while the latter can only make new protoplasm when living matter is included in its food supply, the green plant, by the exercise of its vital chemistry, can transform that which is not living into that which is life-possessing.

The green plant in other words, raises non-living into living matter, while the animal can only transform living matters into its like. This is why the plant is called a constructive organism, while the animal is, contrariwise, named a destructive one. The result of the plant's existence is to build up, that of the animal's life is to break down its substance, as the result of its work, into non-living matter. The animal's body is, in fact, breaking down into the very things on which the green plant feeds. We ourselves are perpetually dissipating our substance in our acts of life and work into the carbonic acid, water, ammonia, and minerals on which plants feed. We "die daily" in as true a sense as that in which the apostle used the term. And out of the debris of the animal frame the green plant builds up leaf and flower, stein and branch, and all the other tokens of its beauty and its life.

If, then, an animal can only live upon living matter--that is to say on the bodies of other animals or of plants--with water, minerals and oxygen gas from the air thrown in to boot, we might be tempted to hold that in such distinctive ways and works we had at last found a means of separating animals from plants. Unfortunately, this view may be legitimately disputed and rendered null and void, on two grounds. First of all, the mushrooms and their friends and neighbors, all true plants, do not feed as do the green tribes. And secondly, many of the green plants themselves can be shown to have taken very kindly to an animal mode of diet.

A mushroom, thus, because it has no green color, lives upon water, oxygen, minerals, and organic matter. You can only grow mushrooms where there is plenty of animal matter in a state of decay, and as for the oxygen, they habitually inhale that gas as if they were animals. Non-green plants thus want a most characteristic action of their green neighbors. For the latter in daylight take in the carbonic acid gas, which is composed of carbon and oxygen. Under the combined influence of the green color and the light, they split up the gas into its two elements, retaining the carbon for food and allowing the oxygen to escape to the atmosphere. Alas! however, in the dark our green plant becomes essentially like an animal as regards its gas food, for then it is an absorber of oxygen, while it gives off carbonic acid. If to take in carbonic acid and to give out oxygen be held to be a feature characteristic of a plant, it is one, as has been well said, which disappears with the daylight in green plants, and which is not witnessed at all in plants that have no green color.

So far, we have seen that not even the food of plants and animals can separate the one kingdom of life from the other. The mushroom bars the way and the green plant's curious behavior by night and by day respectively, in the matter of its gas food, once more assimilates animal life and plant life in a remarkable manner. Still more interesting is the fact, already noticed, that even among the green tribes there are to be found many and various lapses from the stated rules of their feeding. Thus what are we to say of the parasitic mistletoe, which, while it has grown leaves of its own, and can, therefore, obtain so much carbon food from the air on its own account, nevertheless drinks up the sap of the oak or apple which forms its host, and thus illustrates the spectacle of a green plant feeding like an animal, on living matter? Or, what may we think of such plants as the sundew, the Venus' fly trap, the pitcher plants, the side saddle plants, the butterworts and bladderworts, and others of their kind, which not only capture insects, often by ingenious and complex lures, but also digest the animal food thus captured? A sundew thus spreads out its lure in the shape of its leaf studded with sensitive tentacles, each capped by a glistening drop of gummy secretion. Entangled in this secretion, the fly is further fixed to the leaf by the tentacles which bend over it and inclose it in their fold. Then is poured out upon the insect's body a digestive acid fluid, and the substance of the dissolved and digested animal is duly absorbed by the plant. So also the Venus' fly trap captures insects by means of its leaf, which closes upon the prey when certain sensitive hairs have given the signal that the animal has been trapped. Within the leaf the insect is duly digested as before, and its substance applied to the nutrition of the plant. Such plants, moreover, cannot flourish perfectly unless duly supplied with their animal food. Such illustrations of exceptions to the rule of green plant feeding simply have the effect of abolishing the distinctions which the diet question might be supposed to raise between animals and plants. We may return to the sundews and other insect catchers; meanwhile, I have said enough to show that to the question, "Can we separate animals from plants?" a very decided negative reply must be given. Life everywhere exhibits too many points of contact to admit of any boundary line being drawn between the two great groups which make up the sum total of organic existence.--_Illustrated London News._

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THE RECOVERY OF SILVER AND GOLD FROM PLATING AND GILDING SOLUTIONS.

In view of the rapid development and extension of the methods of electro-plating with silver and gold, and of the large amount of spent liquors containing silver or gold thus produced, it has long been desirable to find methods by which these metals can be recovered from the spent liquors. The processes hitherto adopted generally necessitate the tedious and unpleasant evaporation of the cyanide liquors, or else involve a series of chemical operations which are somewhat difficult to carry out, so that actually the used-up baths are sold to some firm which undertakes this recovery as a particular branch of its business.

A process invented by Stockmuir and Fleischmann, and worked out by them in the chemical laboratory of the Bavarian Industrial Museum, is, however, exceedingly simple, and is employed in many establishments.

In order to remove silver from a potassium cyanide silver solution, it is only necessary to allow a clean piece of plate zinc to remain in the liquid for two days; even better results are obtained by the use of iron conjointly with the zinc. In the first case, the silver often adheres firmly to the zinc, while in the second it always separates out as a powder. It is then only necessary to wash the precipitated powder, which usually contains copper (since spent silver solutions always contain copper), dry it, and then dissolve it in hot concentrated sulphuric acid, water being added, and the dissolved silver precipitated by strips of copper. The silver thus obtained is perfectly pure. If the amount of copper present is only small, it can usually be removed by fusing the precipitated powder with a little niter and borax.

In this way a spent silver bath was found to contain per liter

1st experiment 1.5706 grms. 2d " 1.5694 " ------ Mean 1.5700 "

The presence of silver could not be qualitatively ascertained in the residual liquor.

Although sheet zinc, or zinc and iron sheets, serve so well for the precipitation of silver, they cannot be employed for the recovery of gold. The latter separates out in such a case very incompletely and as a firmly adhering lustrous film in the zinc. On the other hand, finely divided zinc, the so-called zinc dust, is an excellent substance to employ for precipitating gold quantitatively and in the form of powder from spent cyanide liquors. When zinc dust is added to a spent gold bath and the liquid periodically stirred or shaken, all the gold is precipitated in two or three days. The amount of zinc to be added naturally depends on the quantity of gold present. Freshly prepared gold baths for gilding in the cold contain on the average 3.5 grms. gold per liter, while those used for the hot process contain 10.75 grms. To precipitate all the gold in the original bath, 1.74 grms. or 0.37-0.5 grms. zinc dust would be necessary, and, of course, a much smaller quantity would be sufficient for the spent liquors. Since the precipitation takes place more rapidly when an excess of zinc dust is present, it is generally advisable to add ¼ or at the most ½ kilo, of zinc dust to every 100 liters of solution.

The precipitated gold, which contains zinc dust and usually silver and copper, is washed, freed from zinc by hydrochloric acid, and then from silver and copper by nitric acid and thus obtained pure.

A spent bath treated in this way gave the following amounts of gold per liter:

1st experiment 0.2626 2d " 0.2634 Mean 0.2630 grms.

The presence of gold in the residual cyanide solution could not be qualitatively detected. The potassium cyanide of the solutions obtained by this process should be converted into ferrocyanide by heating with ferrous sulphate and milk of lime, since this substance is not poisonous and can therefore be got rid of without danger. It would, however, be more economical and, considering the large amount of cyanide present, more profitable to work it up into Prussian blue.

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