THE FARMER AT HOME. 163 



tie surface. Tnat portion of the water which soaks into the earth 

 laving passed through a sufficient thickness of porous strata, either 

 >y ascent or descent, will have all extraneous mixtures detained, and 

 )ecome clear spring water. It should he observed, that filtration 

 :an only produce transparency, by arresting such particles of matter 

 is are in a state of mechanical mixture with the fluid, for any matter 

 \vhich is held in chemical solution in the fluid will pass with it, 

 through the pores of the most minute filter, unless the substance of 

 the filter itself should have a greater affinity for such matter than the 

 fluid which contained it. In this case, a new combination will be 

 formed, and the matter in solution, leaving the fluid, will be taken 

 up by the filter, not simply because the passages are too small to per- 

 mit its particles to pass, but on account of the superior elective attrac- 

 tion between the substance of the filter and the dissolved matter. 



Filtration, on this principle, cannot continue to produce a natural 

 spring for any great length of time ; because, by the constant addition 

 of matter, the filter will at last become saturated with it, or choaked 

 up. In applying this reasoning to springs, we shall find a reason 

 why so few springs produce pure water, although it is always trans- 

 parent. In reality, the great natural filters which produce springs, 

 are almost always on an opposite principle, viz., that the sub- 

 stance which composes the filter has a great affinity for the water, 

 and its particles are thereby taken slowly in solution, and carried off 

 at the same time that the extraneous matters, which are only in 

 mixture with the water, are detained in the pores of the filtering 

 strata ; thus we find few springs which have not some mineral held 

 in solution by the water, although invisible to the eye ; and in cases 

 where heat is generated in making the new combination we have 

 spoken of. hot springs will be produced. The most common mineral 

 taint which water receives in its natural filtration, is sulphate of lime 

 or plaster of Paris ; this renders the water hard, as it is called, so 

 that it will not produce a lather with soap, but curdles it. Sulphate 

 of iron or vitriol is also frequent in springs. Add to this, that in great 

 towns, the drainage water which soaks into the earth is contaminated 

 by animal matters as well as vegetable, and in such an offensive state, 

 that the filtration through the soil can scarcely restore its purity. 



Gravel, in thick beds, is the most perfect natural filter ; and 

 instances may be met with, of springs from gravel producing water 

 very nearly as pure as distilled water. Sand, when white, such as 

 that of the seashore, is also very good ; but if colored, it generally 

 contains iron ; and where the color is deep, the iron is often in such 

 excess, that it will be communicated to the water in passing through 

 it. Beds of sandstone filter extremely well, and also some porous 

 limestone. 



FIXED STARS. The universe, so far as human observation 

 has extended, consists of infinite or boundless space, in which are 



