78 LESSONS IN CHEMISTET. 



its naked surface exhibited no trace of vegetable life. But 

 every one must have observed how the surface of the rocks 

 of a country gradually crumbles into powder, which, of differ- 

 ent degrees of fineness, accumulates in the sheltered hollows, 

 or is washed down upon the plains.* 



1 11 . In nature everything is subject to that change of form 

 I which we designate decay. The period at which this takes 

 I place is different for different forms of matter, but the differ- 

 I ence is merely one of time. The flinty rock, the hardest 

 ^ metal, the densest wood, gradually change their forms. The 

 ; great agents in producing these changes are the gases which 

 \ exist in the atmosphere, and which, dissolved in the water 

 \ that falls from the clouds penetrate into the earth and rest 



\ upon the surface of our buildings and the face of the rocks 

 I in our fields. By far the most energetic of these gases is 

 { carbonic acid, which penetrates into the crevices with the 

 rain, and enables it to dissolve the lime, magnesia, and other 

 substances of which the rocks are composed; these are gra- 

 dually washed out, the cohesion of the undissolved portions 

 '' being destroyed, and are easily carried away by the storm 

 V and rains and deposited upon the fields. 



112. The inferior orders of plants assist in producing these 

 changes. They are pioneers of vegetation, employed by 

 nature in preparing a store of materials for those plants 

 required as food by man. Thus, no sooner does the rain 



* The rapidity with which a rock crumbles to powder by the influence 

 of the atmosphere depends upon its density and power of absorbing 

 moisture. In relation to this subject, I may here give some interesting 

 examinations of Irish rocks, made by Mr. "Wilkinson, architect to the 

 Poor Law Commission. According to his experiments, 



The chalk of Antrim weighs, per cubic foot, 160 lbs. and absorbs 3 



lbs. of water. 

 Shaly Calp weighs, per cubic foot, 160 lbs. and absorbs from 1 to 



4 lbs. of water. 

 The average weight of Sandstone, per cubic foot, is 145 lbs. and it 



absorbs 1 to 6 J lbs. of water. 

 Some Sandstones absorb scarcely any water. 

 The average weight of Granite, per cubic foot, is 170 lbs. 

 The Granite, of Glenties in Donegall, absorbs 4 lbs. of water, and 



that of Newry and Kingstown, ^ lb. per~cubic foot. 

 The average weight of Basalt, per cubic foot, is 178 lbs. and it 



absorbs less than ^ lb. of water, per cubic foot. 

 The average weight of Clay Roofing Slate, per cubic foot, is 177 lbs. 



it absorbs less than ^ lb. of water. 



