4 



KNOWLEDGE 



[January 1, 1900. 



liquid known as " cell-sap." which is water holding in 

 solution various materials which have been taken up 

 from without by the roots and leaves. These materials 

 arc thus brought in contact with the protoplasm which 

 causes them to undergo changes in composition that 

 prepare them to be added to the substance of the plant. 

 Thus it is in the protoplasm of the living cells of the 

 plant that those " digestive " processes are carried on 

 ■which Aristotle believed to occur in the soil. We see 

 then that the living cells are microscopic laboratories 

 in which the digestion of the food of the plant is carried 

 on. And now that we know something of the nature of 

 the laboratories *o which the food materials have to be 

 conveyed we can the more easily enquire into the nature 

 and sources of the food supply. 



As we cannot directly observe substances entering or 

 leaving a plant, we must adopt other methods of in- 

 vestigation before we can learn much about the nature 

 of its food. If we can ascertain of what substances a 

 plant is composed, we shall at least know that by som? 

 means or other it has received these substances from 

 outside. For example, when a cigar burns there re- 

 mains an incombustible grey ash. Chemical analysis 

 of this ash shows that it is composed of various sub- 

 stances, one of which is Magnesium. We know then 

 that Magnesium was present in the cigar. If we were 

 acquainted with the method by which the cigar was 

 manufactured we might go back a step further and 

 conclude that Magnesium was contained in the tobacco 

 leaf from which the cigar was made, and therefore in 

 the food of the tobacco plant. This is an illustration 

 of the actual method employed when we wish to find 

 out what are the substances which a plant acquires 

 from the outside world. The whole of a well-growii 

 plant is thoroughly dried at a high temperature, which 

 is, however, not high enough to bui-n it. After being 

 carefully weighed, the dried plant is burned in such a 

 manner that not only the ash, but also the gases given 

 off during the combustion, are collected. These are 

 then weighed and analysed. The gases, which weigh 

 considerably more than the ash, are found to be Carbon 

 dioxide (COr,), water vapour (H;0), and Nitrogen. Wo 

 should then conclude, on chemical grounds, "that our 

 plant contained Carbon, Hydrogen, and Nitrogen. The 

 ash may contain a great number of elements, many of 

 which are found in only a few plants; of these the 

 following are almost always present — Calcium (a con- 

 stituent of chalk). Magnesium (found in many Lime- 

 stones,) Iron, Potassium, Sulphur, Phosphorus, Chlor- 

 ine, Silicon, and Sodium, all of which are found in 

 the soil. At the same time we should learn that our 

 plant contained Oxygen. (It would take us too far to 

 consider the reasons for this conclusion.) 



It will be interesting to place side by side the results 

 of such an analysis of Green Peas and Clover Hay : § 



i;B£en peas. ri.ovEii hav. 



< 'arhon - -tR..') per pent. 47.4 ))er cent. 



Hydrogen - 0.2 „ „ 5.0 



Oxygen - 40.0 „ „ 37,8 „ 



Nitrogen - 4.2 „ „ 2 ] 



Ash* - - 3.1 „ „ 7.7 ' '.'. 



100.0 100.0 



We cannot discuss here all the information that is con- 

 tained in these figures. We notice, however, that the 

 proportions of each constituent differ in the two cases, 

 and this is true in all cases, even for similar parts of 

 two plants of the same kind growing side by side in the 



Vine's " Physiology of Plants." 

 * All the mineral constituents are here inclucled. 



same soil. And further, we see that in each case the 

 Carbon weighs more than any other constituent. This 

 is always the case in herbs and in the soft parts of 

 woody plants. 



This method of analysis shows us what are the ele- 

 ments which a plant takes in from without. But we 

 learn from it nothing concerning the forms in which 

 these elements must be combined before they can be 

 received by the plant. And further, it gives no in- 

 formation as to whether any of the elements in the 

 substance of the plant may be more necessary for the 

 general welfare of the body than any others; in other 

 words, we cannot 'liy mere analysis find out which are 

 the necessary elements of the food as distinguished from 

 those whch can be dispensed with without injury to the 

 life of the plant. 



To attack such problems as these we must have re- 

 course to other more direct methods of investigation. 

 The most important of these is the method known as 

 " water-culture." Plants are grown, sometimes for con- 

 siderable periods, with their roots immersed in water in 

 which various mineral salts are dissolved. "Sand-culture" 

 is merely a modification of " water-culture " ; in this 

 the roots are in pure sterilised sand, to which such so- 

 lutions as are used in water-cultures are added. From 

 a great number of experiments of this kind it is possible 

 to find the composition of a solution in which a plant 

 will grow, for a time at least, as well as if its roots 

 were in the soil under natural conditions. Such a 

 solution, in which most green plants are able to find 

 all the necessary elements of their food, has the follow- 

 ing composition: — 



Distilled Water . - . - 1,000— l,50t) grammes. 



Potassium Nitrate (Saltpetre) ... i.o „ 



Magnesium Sujpliate (KjiBom Salt) - - 0.5 „ 



Calcium Sulphate ..... 0.5 „ 



Calcium Pliosphate or Potassium Phosphate - 0.5 „ 



A soluble Salt of Iron ..... A trace. 



This solution contains all the elements which the roots 

 of a green plant must find in the soil in order that its 

 growth may be healthy. Other mineral elements found 

 in plants are not as a rule essential constituents of the 

 food. The proportions in which these substances are 

 presented to the roots may vary without affecting the 

 growth of the plant, for the roots have the power of 

 regulating the quantities of each substance absorbed, so 

 that none enters in excess. An important point to be 

 noticed in the composition of this nutrient solution is 

 that Carbon is not present in it in any form what- 

 ever. A green plant is able to obtain all its Carbon 

 from the atmosphere, though, as we shall see later, it is 

 quite possible that in Nature some of it is absorbed by 

 the roots. 



EXPLOSIONS IN COAL MINES. 



By John Mills. 



A MOST important example of an industrial result 

 depending upon pure induction from abstract science is 

 afforded by the Miner's Safety Lamp. The processes 

 which, in the course of ages, have resulted in the pro- 

 duction of coal — the presei'ved matter of primeval 

 forests — the remains of a vegetable world, have also 

 yielded an abundance of inflammable gas. This gas, 

 known to miners as " fire damp " and to chemists as 

 carburetted hydrogen, accumulates in the cavities and 

 fissures of the coal itself, and of the adjacent strata; it 

 is identical in composition with marsh gas, which causes 

 the phenomenon known as " will-o'-the-wisp." 



Fire damp, the scourge of the miner, more dreadful 



