518 REPORTS ON THE STATE OF SCIENCE, ETC. 



places along the sea-coast, and it is on this account that sand-banks are formed in 

 rivers and that the sea-shore more often than not consists of sand. 



Sandstone. — Sand is found in many places mixed up with pebbles of various 

 sizes — how are such rounded pebbles produced, do you suppose? If you have 

 been on the sea-coast where there is a shingle beach, you will probably be able 

 to account for the rounding of the pebbles. What are gravel pebbles like 

 inside — do they in any way resemble sand ? 



Hard rocks are of frequent occurrence which are obviously formed of sand 

 particles stuck firmly together — these are commonly known as sandstones ; they 

 are usually coloured more or less — yellow, brown or even bright red. Flint, chert 

 and quartz are sohd, almost glass-like rocks, which when broken into small pebbles 

 give a material like sand. 



Clay. — In many places, soft rocks are found which are more or less easily 

 split up into slabs or sheets ; these are known as shales or slaie rock. If the 

 fine powder formed by grinding them be mixed with water, it forms a more or 

 less siticky, clay -like mass. 



Limestones. — Rocks which yield lime when burnt are very generally met 

 with together with sand and clay; they vary much in character according to 

 the district, some being soft like chalk, others hard and crystalline like mountain 

 limestone. The limestones are always full of fossils ; chalk under the microscope 

 appears to consist almost entirely of shell-like remains. 



Igneous rocks. — Sandstone, clay and limestone axe known as sedimentary 

 rocks — there being complete proof that they have been deposited as sediments 

 from water. 



A fourth class of rock includes all rocks which have cooled down from the 

 fused state. Granite is one of the most characteristic of these rocks and is 

 well known, as it is much used as an ornamental stone for building. 



Everyone should be familiar with these common rocks and take some interest 

 in their history and the wonderful story they tell when properly interpreted : this 

 study should be made almost entirely an outdoor occupation. 



Nature of Limestone. 



In studying starch, we have taken into account things which were known 

 about it and have based experiments on these ; the results have enabled us to 

 arrive at certain conclusions ; our discovery that starch contains carbon and 

 perhaps water was based on the study of the changes which it undergoes when 

 heated and when burnt in air. We were led on to study the changes which 

 metals undergo when burnt and to discover that the earthy substances into 

 which they are converted are compounds of the metals with P^ire air. We were 

 able to take away the Fire air from the metal in some of the earths by means 

 of carbon. In every case a change was effected — we arrived at our knowledge 

 of the nature of the subject by studying a change in which it was concerned. 

 Can this method be applied to the study of soil materials — in appearance they 

 resemble closely the earths obtained by burning metals — are any of them known 

 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 



