OBJECTS.] INTRODUCTORY. 35 



some oil into the water, it floats, and if we pour some 

 coloured spirit in carefully, it also floats ; while treacle 

 and quicksilver sink to the bottom, just as the iron- 

 filings do. 



We saw that the iron-filings sank, because iron is 

 heavier than water. Here is a piece of the thin tinned 

 sheet-iron that they make tin boxes of. What will 

 happen if we drop it into the water? It is heavier 

 than water, bulk for bulk, and therefore it will sink as 

 you see it does. 



But now here is a " tin" canister made of this very 

 same tinned sheet-iron. We drop that into the water, 

 and you see it does not sink at all, but floats at the 

 top as if it were made of cork. Here is a perplexity. 

 We were sure just now that iron is heavier than water, 

 and here is an iron box floating ! Is this an exception 

 to the law ? Not at all ; for what we said was that a 

 thing would float if it were lighter, bulk for bulk, 

 than water. Now let us weigh the tin box, and having 

 weighed it let us next try to find out how much the 

 same bulk of water weighs. This may be done very 

 simply, for the walls of the box are very thin, so that 

 the inside of the box is very nearly as large as the 

 whole box. Consequently, if we fill the box with 

 water, and then weigh the water, we shall find out, 

 very nearly, what is the weight of a bulk of water as 

 great as that of the box. But if we do this, we shall 

 find that the water which was contained in the box, 

 weighs very much more than the box does. So that, 

 bulk for bulk, the box, although it is made of iron, 

 is really lighter than water, and that is why it floats. 



You will all have heard of the iron ships which are 

 now so common, and you may have wondered how it 



