686 SCIENCE PROGRESS 



all gritty matter and dirt, is used all the world over in making 

 whitewash, whilst when mixed with oil it forms putty. We have 

 some reason to be grateful, therefore, to the Globigerina for 

 producing so universally useful a material as chalk. But it is a 

 substance which sometimes makes itself awkward. There are 

 few districts in which the water does not contain more or less 

 dissolved limestone, being thereby rendered " hard," as it 

 is termed : such hard water, we know, is less suitable than 

 rain water for washing purposes and is often productive of 

 considerable difficulty when used in steam boilers. 



How is such a substance to be dealt with to discover its 

 nature ? Is no clue to be found by considering what is commonly 

 known about it ? 



A man's character is not be found out by merely looking at 



him : his actions must be studied. To determine the character 



of a substance, its capabilities must be inquired into. Every one 



knows that limestone gives lime when burnt — the name implies 



as much. Some profound change must be involved — the lime is 



so very different from the limestone. On the Kentish coast but 



a few miles away, twice during every twenty-four hours, the 



chalk cliffs are wetted by the waves — yet nothing happens. But 



every bricklayer's labourer who is called upon to make mortar has 



learnt the trick of reducing lime to the finest of powders without 



any effort to himself — by merely pitching a pailful or two of 



water upon it : the man who made the observation originally and 



applied his discovery in practice must have been a transcendent 



genius ; he scarcely wore trousers, but had he done so he would 



have realised that he could put his hands quietly into his pockets 



and look on at the water doing his work for him and doing it far 



more effectively than he could himself. Yet the beauty of the 



process is rarely appreciated. Carry it out for yourselves and 



see what happens. The water is greedily absorbed : then, after 



a time, the wetted lime grows hot, clouds of vapour are given 



off — and gradually the lumps fall into an impalpable dry powder, 



riven in every direction by the expansive force of the steam. 



The lime is said to be " slack" or slaked — its thirst is satisfied. 

 But where does the water go ? How is so marvellous a change 

 to be explained ? The time should be near at hand when even 

 the bricklayer's labourer will ask such questions. I trust many 

 of you take real interest in the spectacle and desire to under- 

 stand what has happened. If I ask you to suggest some means 



