Dinbur Castle, its Gardens and Gardeners. 609 



After Sandy had finished the few remarks he had intended 

 to make, he told them, if any of them had a question to ask 

 respecting the subject they had been hearing, he would en- 

 deavour to answer it. 



Walter Glenesk said that he had learned more about 

 oxygen during the short time they had been together than 

 ever he did before in his life; but, if he recollected right, he 

 had read somewhere about plants giving it out, and if he could 

 give him any information about it he would take it kindly. 

 " Yes," said Sandy, " they give it out and take it in too. We 

 are informed, by chemists and vegetable physiologists, that the 

 dark substance that remains after manure is rotten, and which 

 gives the black colouring to the earth, is called humus ; the 

 oxygen of the atmosphere combining with humus, food is pre- 

 pared for plants, carbonic acid is formed, water absorbs it, it is 

 again decomposed by the plant, the carbon is fixed, and oxygen 

 given off." 



"Then it must be of great service," said Colin Forbes, 

 " to vegetables, to keep the earth in such a state that the 

 oxygen of the atmosphere may have free access to their 

 roots." — " Yes," l'eplied Sandy, " it is of great importance to 

 keep them in such a state that they may freely come in con- 

 tact with the air that surrounds them; and frequent hoeing, 

 properly done, is of greater use to vegetables than many persons 

 are aware of: by stirring the earth often, oxygen combines 

 with the carbon of the soil, and food is provided for vegetation. 

 It may not be the only way in which plants are nourished, which 

 we may have an opportunity of showing some other time." 



" Donald Blamart," said Bauldy, " used to say that the only 

 use of hoeing was to kill weeds, and I have often wondered, 

 since I came here, why I was set to hoe crops and no a weed 

 amang them; but I think I understand the reason noo. But 

 I would like to ken whar a' that gas came frae that filled sae 

 mony bell-glasses." — " Oxygen," said Sandy, " has a powerful 

 attraction for a great number of simple substances, and the act 

 of combining with it is called oxidation, and the compounds 

 formed in this manner are divided into acids and oxides. For 

 instance, when 1 part of carbon and 2 parts of oxygen combine, 

 carbonic acid is formed ; again, when oxygen and hydrogen 

 combine in certain proportions, water is formed ; and, in 

 chemical language, water is called protoxide of hydrogen. The 

 affinity of iron for oxygen is also very great. When iron is 

 heated to redness in the open air, it absorbs oxygen rapidly, 

 and is turned into black scales, called the black oxide of iron, 

 better known in some places by the name of " smiddy aise." 

 There is also a metal called manganese, which combines readily 

 with oxygen ; this combination is commonly called the black 



3d Sen— 1842. XII. b R 



