196 REPORTS ON THE STATE OF SCIENCE.—1917. 
tin? You can easily try. Magnesium, in the form of ribbon, burns very 
easily—study the change carefully. And try if silver can be burnt. 
Having previously contrasted iron with iron rust by determining their 
densities, it will be well in the case of other metals to contrast each of the 
products with the metal from which it is formed and to draw up a tabular 
statement of the results arrived at. It will be well, instead of making all the 
substances, to inquire if you cannot obtain the various burnt metals and at 
the same time to collect information as to the use that is made of them and 
of their market value in comparison with that of the metals, At the same 
time, it will be well to inquire how the metals are made. 
Such a comparison affords most instructive results—in every case, the metal 
affords an earthy product: some of the earths are relatively light, others 
heavy—some are coloured, others colourless; how do they behave towards 
water—have they any taste? 
All this time, the snow formed on burning phosphorus—which is certainly 
not at all like a metal—has been left out of consideration : it should therefore 
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; and if one be obtained collect it and examine it in com- 
parison 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 our 
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 to 
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 
