NECESSITY OF CERTAIN CONDITIONS FOR NUTRITION. 7? 



corn plants appear, because their seeds cannot attain maturity 

 unless supplied with the constituents of those matters. 



When we find sea plants near our salt-works, several hundred 

 miles distant from the sea, we know that their seeds have been 

 carried there in a very natural manner, namely, by wind or by 

 birds, which have spread them over the whole surface of the 

 earth, although they grow only in those places in which they find 

 the conditions essential to their life. 



Numerous small fish, of not more than two inches in length 

 (Gasterosteus aculeatus), are found in the salt-pans of the gradu- 

 ating-house at Nidda (a village in Hesse Darmstadt). No living 

 animal is found in the salt-pans of Neuheim, situated about 18 

 miles from Nidda ; but the water there contains so much car- 

 bonic acid and lime, that the walls of the graduating-house are 

 covered with stalactites. Hence the eggs conveyed to this place, 

 by whatever cause, do not find the conditions necessary for their 

 development, although they did so in the former place. 



How much more wonderful and inexplicable does it appear, 

 that bodies, remaining fixed in the strong heat of a fire, have un- 

 der certain conditions the property of volatilizing and, at ordinary 

 temperatures, of passing into a state, of which we cannot say 

 whether they have really assumed the form of a gas or are dis- 

 solved in one ! Steam or vapors in general have a very singu- 

 lar influence in causing the volatilization of such bodies, that is 

 of causing them to assume the gaseous form. A liquid during 

 evaporation communicates the power of assuming the same state 

 in a greater or less degree to all substances dissolved in it, 

 although they do not of themselves possess that property. 



Boracic acid is a perfectly fixed substance ; it suffers no 

 change of weight appreciable by the most delicate balance, when 

 exposed to a white heat, and therefore it is not volatile. Yet its 

 solution in water cannot be evaporated by the gentlest heat, with- 

 out the escape of a sensible quantity of the acid with the steam. 

 Hence it is that a loss is always experienced in the analysis of 

 minerals containing this acid, when liquids in which it is dissolved 

 are evaporated. The quantity of boracic acid which escapes 

 with a cubic foot of steam, at the temperature of boiling water, 

 cannot be detected by our most sensible re-agents; but neverthe- 



