ATTRACTION. 173 



of one to one leg, and the south pole of the other to the other leg of 

 the syphon tube. Circles of tallow being formed on glass plates, so- 

 lution of nitrate of silver was placed within, and a circular piece of 

 zinc in the centre ; the precipitation of silver was much more active 

 towards the north, and the oxide of zinc inclined to the south ; a 

 strong magnet being brought within two inches of a plate prepared, as 

 before, the precipitation took place in one fourth of the time, that it 

 did on the plates that were beyond its influence.* 



* * * * -x- 



We have now taken a preliminary view, perhaps sufficiently ex- 

 tensive and detailed, of the general doctrines of chemistry. This was 

 indispensable, to enable us to understand the history of particular bo- 

 dies, which is to follow ; and in giving it, I have endeavored, as far 

 as practicable, to avoid anticipation ; still it is possible that some pas- 

 sages may be unintelligible to a beginner ; as they are, however, not 

 numerous, they may be omitted in the first reading, and being mark- 

 ed in the margin by a pencil, they can be examined again at a more 

 advanced stage of the subject, when the pupil has become more 

 familiar with chemical facts and reasoning. 



The preceding account of the general doctrines, although proba- 

 bly sufficient for an introduction, is far from being complete, and ad- 

 ditional illustrations will be given, when the proper facts come in 

 our way. 



Before proceeding to the history of particular bodies, it will be 

 useful to say something of the rules of philosophising, and of the ap- 

 paratus and operations. 



I. RULES OF PHILOSOPHISING. LIMITS OF HUMAN REASON. 



1. GOD IS THE FIRST CAUSE OF EVERY THING. 



(a.) Ml our observations, experiments and reasonings, make us 

 acquainted only with second causes* 



(b.) Theproximate cause of an effect, is the one immediately an^- 

 tecedent to the event, or which is principally operative in produc- 

 ing it. 



(c.) To every proximate cause, there may be another proximate 

 cause, and to that cause another ; but the series will end at last in 

 the power of the Creator, in immediate agency ; and this will still 

 be the fact if we discover ever so many proximate causes, constitu- 

 ting a series or chain apparently endless. 



(d.) When we have classified similar phenomena, and have dis- 

 covered their modus operandi ; we say that we have found out the 

 law that governs them ; but still this harmony of facts and operations, 

 we must trace to the same source. 



* Am. Jour. Vol. XVI, p. 262, and Ann. de Chim. et de Physique. 



