288 APPENDIX. 



Heat has a tendency to separate the particles of all 

 bodies from each other. Hence nothing more is necessary 

 to eifect the decomposition of many bodies than to apply 

 heat, and collect the substances which are separated by 

 that means. 



It is evident that water exists in the atmosphere in 

 abundance, even in the driest season, and under the clear- 

 est sky. There are substances which have the power of 

 absorbing moisture from the air, at all times, such as the 

 fixed alkalies, (potash and soda,) and sulphuric acid, the 

 latter of which will soon absorb more than its own weight 

 of water from the air, when exposed to it. Fresh-burnt 

 lime absorbs it rapidly ; and earth that has been freshly 

 stirred absorbs it in a much greater degree, at night, than 

 that which is crusted and compact. Hence the impor- 

 tance of stirring the soil among tillage crops, in time of 

 drought. 



Bishop Watson found, that even when there had been 

 no rain for a considerable time, and the earth v/as dried 

 by the parching heat of summer, it still gave out a consid- 

 erable quantity of wat^r. By inverting a large drinking- 

 glass on a close-mown grass plat, and collecting the vapor 

 which attached to the inside of the glass, he found that an 

 acre of ground dispersed into the air about 1,G00 gallons 

 of water in the space of twelve hours, of a summer's day. 



Lavoisier has explained solidity thus : " The parti- 

 cles of all bodies," says he, " may be considered as sub- 

 ject to the action of two opposite powers, repulsion and 

 attraction, between which they remain in equilibrio. So 

 long as the attractive force remains stronger, the body must 

 continue in a sta.te of solidity ; but if, on the contrary, heat 

 has so far removed these particles from each other as to 

 place them beyond the sphere of attraction, they lose the 

 cohesion they before had with each other, and the body 

 ceases to be solid." 



Aeriform substances (gases and vapors) are called 

 elastic, because they are all capable of being reduced into 

 a smaller compass by pressure, and of expanding again to 

 their usual volume whenever the pressure is removed. 

 Thus atmospheric air may be so compressed, that 128 

 volumes may be forced into a space usually occupied by 

 one volume, and the greater the compression the more will 

 its elasticity be increased. It is on this principle that the 

 air-gun is constructed. — Parke. 



