Take three big glass jars, each with one litre of distilled water, and label them A, B, and C. Into A put nothing further, into B put the following salts, which have been weighed out carefully either by you or by a chemist:-- Potassium nitrate 1 gramme Calcium sulphate 1/2 „ Sodium chloride 1/2 „ Magnesium sulphate 1/2 „ Calcium phosphate 1/2 „ then add to C all these salts, and also one or two drops of a dilute solution of iron chloride. Into the jars fit corks which are split, with a hole in the centre, and pack a plant into each with the part of the stem between the corks wrapped round with cotton wool (~see~ fig. 10), and so fix the plant that its roots are in the solution and its stem and leaves in the air[2] (~see~ fig. 11). Wrap black cotton or paper round the jars so as to keep the roots dark as they would be in the soil. [2] Weigh the plant, which you are putting in jar C, carefully, and keep a record of its weight for future use (~see~ p. 18). [Illustration: Fig 10. Plant packed in split cork. (~h~) Hole in cork; (~c~) cotton-wool packing the stem.] Do not use too small vessels; in fact, if you had bigger jars and took double quantities of everything it would be better. You may make the experiment more complete by preparing a whole series of solutions with one of the salts left out each time. In this way you would be able to see the effect of the different elements on the growth of the plants, and you would find nitrates are very important. Put a plant, similar to the one you are experimenting with, into a pot of soil or the garden, and keep it well watered. This is called the “control plant.” Very soon you will find that the plant in jar A (the one with only distilled water) is not growing so fast as the others, and after a time will die off completely. The one in jar C with all the salts, on the other hand, should grow quite as well as the control plant in the garden, which you should take as the standard. The plant in jar B, when it has everything but iron, should act in a curious manner. At first it should grow all right and outlive the one in distilled water, but after a time its leaves should get paler, till the new ones formed are quite yellow instead of green, and soon after this the plant will die. If, however, you add two drops of the iron solution before it dies, it may recover, become green again, and go on living. It turned a whitish yellow colour because there was no iron in its supply of salts and water. Just as when people get pale and white the doctor orders them iron, so it is necessary for plants to have iron when they begin to lose their green colour. Later on you will find how very important the green colour is, for without it they cannot grow (~see~ Chap. VI.).[3] [3] This experiment is sometimes difficult to manage successfully, though it appears so simple. Great care should be taken not to overdose the plant with iron.