22 Pioneer Labourers 



oxygen to rust iron faster than it could do alone. But 

 when it works on its own account it is by combining 

 with such substances as lime, potash, soda and mag- 

 nesia, which it makes much more soluble than they 

 were before. 



Some rocks are said to be impervious, or 'water- 

 proof,' but this only means that they allow water to 

 enter so very slowly that, unless they are actually 

 soaking in it for some time, hardly any is taken up. 

 And so, in a similar way, some minerals are called in- 

 soluble, which again means only that pure water has 

 very little effect upon them, dissolves them so very 

 slowly that it hardly seems to do so at all. For som& 

 effect it has upon every known mineral, unless it be 

 perhaps upon gold and platinum. But water in nature 

 is never perfectly pure; how can it be, since it dis- 

 solves some, though it may be only a very minute 

 quantity, of everything through, or over, which it 

 passes ? Its dissolving powers are greatly increased, 

 too, by the addition of carbon dioxide — the gas we are 

 now speaking of — which is being constantly produced 

 both in earth and air, by the decay of vegetable matter 

 in the one, and by the lungs ot animals, fires and 

 furnaces, in the other. 



The rain, as it descends from the clouds, washes 

 down with it some of this gas, and if it comes in con- 

 tact with such a rock as limestone, soon makes an im- 

 pression upon it. Chalk, limestone and marble, are all 

 composed of carbonate of Hme, softer or harder, the 

 lime being already united with a certain quantity of 

 carbon dioxide. But in this condition it dissolves so 

 very slowly as to be called insoluble in pure water. 

 When it comes in contact with the gas, however, 



