198 REPORTS ON THE STATE OF SCIPNCE.—1917, 
to undergo change in any characteristic way? What is done with sand? It is 
used along with lime in making mortar and when fused with soda forms glass. 
Clay in admixture with sand is used in making bricks and when burnt with 
chalk yields cement. 
Limestone when burnt is changed into lime; in the form of soft chalk 
or preferably of lime, it is applied to the soil as manure. 
Apparently, all undergo change; limestone, however, is changed when heated 
alone and therefore seems to offer the simplest case for study. 
A series of experiments might follow, on lines like those indicated on 
pp. 355-359 and 444-448 of my ‘ Teaching of Scientific Method’ (Macmillan & 
Co., Ltd.), leading up to the discovery of the compound nature of limestone. 
Limestone has thus been resolved into two substances—solid lime and a gas: 
although not itself an earth like any of those formed on burning metals, the 
lime obtained from it is very similar in appearance at least to the earths 
which are formed from some of them; as to the gas, being colourless, it is not 
easily compared with other gases. What are the properties of the gases you 
have dealt with thus far? Of the two gases in air, one, you know, promotes 
combustion, the other does not; the gas you obtained by burning carbon by 
means of red lead and copper scale was heavier than air and more soluble in 
water than air and a taper would not burn in it. On testing the gas from lime- 
stone, you find that it resembles the latter gas rather than air. But you have 
discovered that the gas from limestone can be reconverted into limestone stuff. 
Does the gas prepared from carbon at all resemble it in this respect? On making 
the experiment you find it does; indeed you cannot distinguish between the 
two—they are the same material. Think what a momentous discovery you 
have made! That carbon is an important constituent not only of vegetable 
and animal matter but also of the earth limestone—it seems to be every- 
where, in some cases in an unburnt, in others in a burnt state. You may ask, 
how comes it to be in limestone—in a burnt state? What is limestone com- 
posed of? Chalk, the form which you have examined, consists of the remains 
of minute shells—shells are of animal origin—are all shells alike in composition? 
Such reflections should lead you to study a variety of shells, salt-water, fresh- 
water and land shells, the shells of birds’ eggs. 
In the course of the experiments with limestone, it has been discovered 
that the gas which is a constituent of limestone stuff is present in minute pro- 
portion in the air. How does it get there? You know that it is formed by 
the combusion of coal, wood, &c. But as we are kept warm by our food and 
it is probable that it is more or less burnt up in our bodies and that the air 
we breathe in is used for the purpose, may it not be that the gas is also given 
out by us? Try to find out by contrasting ordinary air with expired air. fee 
also if the gas be given off by animals, such as mice, by caterpillars feeding on 
green leaves, by snails, &c., by keeping these under a bell jar through which 
air is passed after scrubbing it free from the gas by means of lime. Also en- 
deavour to find out if air be concerned in the germination of seeds by ascertain- 
ing if they germinate in air over water and whether the air be affected, and 
also whether as germination takes place the gas be given off. 
Stupy or Acrps. 
Are you not surprised that you have been able to find out so much—and 
especially that whatever you do you are always led, sooner or later, to dis- 
cover something of interest in relation to yourselves? No doubt you are anxious 
to continue your inquiries now that you begin to understand what wonderful 
changes are going on everywhere. 
The gas obtained by burning carbon resembles the product from phosphorus 
and differs from the earths derived from the metals inasmuch as they are 
bath formed from substances which are clearly not metals—but one being a gas 
and the other a solid they are not directly comparable as are the products from 
the metals. Have they any property in common? What property is character- 
istic or the phosphorus snow? Its taste, is it not? Has the gas an acid 
taste? Try! Acids stain coloured clothes, do they not? The colours of flowers 
are very sensitive—make coloured solutions from a variety of flowers and see 
whether they are affected by solutions of the two substances which you are 
