ON SCIENCK IN SCHOOL CERTIFICATE EXAMINATIONS. 517 



be compared with the earths formed from the metals. You have already learnt 

 that its behaviour is somewhat peculiar — what became of it when the phosphorus 

 was burnt over water? If you did not notice, repeat the experiment. What 

 happened to the snow which fell on the tile when the phosphorus was burnt 

 under the glass shade? Can the snow be kept in a closed bottle? Has it any 

 taste? 



It seems then that earths are produced when Fire air is combined with 

 metals — what other combustible substances yield when combined with it is not 

 yet clear : only in one case, that of phosphorus, have you learnt that a sour 

 or acid-forming substance is produced. 



To understand what becomes of food when it is burnt, it is clearly de- 

 sirable to extend the inquiry — carbon is certainly not a metal and there is no 

 evidence yet that any earthy substance is formed when it is burnt, apart from 

 the small quantity of ashes which remains. 



Has it not struck you as remarkable, when you were hearing of the ways 

 in which the various metals were made, that in most cases carbon in the form 

 of anthracite, coal or coke, was used to separate the metal ? The metallic 

 ores are mostly earthy substances and most of the metals are converted into 

 earths by roasting them in air — what then is perhaps the nature of the action 

 which the carbon exercises in separating the metal ? Will it not be well to 

 try experiments with the earths prepared from the metals or with those 

 which afford metals and to heat them with charcoal? In some cases, you obtain 

 the metal easily — what else ? Nothing solid or liquid — perhaps an air or gas 

 is produced. Try ; if one be obtained, collect it and examine it in comparison with 

 air by determining its density, &c. Then see what happens on burning starch in a 

 similar way. After these experiments, there can be no doubt that the carbon in 

 starch is of value as a combustible. 



Plants and Soils. 



Although our food is partly of animal and partly of vegetable origin, ex- 

 cepting fish, poultry and game, the animals we use as food are entirely vegetable 

 feeders : directly or indirectly, therefore, we are dependent on plants for oui 

 food — we could not live on air and water and the soil as they do. The knowledge 

 gained from the experiments you have made enables you already to ask of 

 what use is air to plants — do they breathe as we do ? They are not warm, as 

 we are — nevertheless, it may help them to burn some of their food slowly. 

 What is their food — where do they obtain the carbon which is contained in 

 starch and which we must suppose is a chief constituent of plants, of wood 

 and of all vegetable materials, as they all give more or less charcoal when 

 heated sufficiently strongly ? The use to them of water we can understand t(i 

 some extent, as they are full of watery juices, like ourselves. Of what use 

 to them are roots — do they suck all their food out of the soil with their aid ? 

 As roots are peculiar to plants, it does not seem unlikely that this is the 

 case. Considerations such as these make it desirable to know something of the 

 soil. 



To grow plants properly, they must be cultivated ; all spoils are not equally 

 good. What is soil ? The surface crust of the earth. Even in those regions 

 which consist of hard rock, the surface is usually soft soil formed by the 

 gradual decay of rocks imder the influence of the weather. What kinds of soft rock 

 or soil do you know — what kinds of hard rock ? 



The soft soil everywhere is either sand or clay or a mixture of these (loam). 

 You probably know both kinds and are well aware that they are very different, 

 but it is better that you should examine them carefully. Take . . . grams of 

 each, examine them — if possible with a magnifying lens; describe them, con- 

 trast their behaviour, also their behaviour with water, both when wetted witii 

 it and when stirred up with a considerable quantity. Afterwards examine some 

 garden and field soil and see what you can separate by stirring up the soil 

 with water and decanting off the water before the lighter particles hav» 

 settled. 



The separation of sand from clay is always going on in rivers and in many 



