ATTRACTION. 



(ft*.) Instances of definite compounds are innumerable. Thus, 

 sulphate of baryta, whether formed by art, or existing for ages, as a 

 natural production, is composed of baryta 40 parts, and sulphuric acid 

 78 ; and they cannot be made to combine in any other proportion ; 

 if the acid and a solution of the earth are mingled in any different 

 proportions, the ingredient that is in excess will be left untouched. 



Baryta itself is composed of the metal barium 70, and oxygen 8= 

 78, and sulphuric acid of sulphur 16, and oxygen 24=40. Nitrate 

 of potassa (saltpetre) is composed of nitric acid 54, and potassa 48= 

 102, and nitric acid is composed of nitrogen 14, and oxygen 40 =54, 

 and potassa of potassium 40, and oxygen 8=48. 



(oo.) The combining power of all bodies can be expressed by num- 

 bers.* This remarkable fact can be rendered intelligible by the fol- 

 lowing instance. In the composition of water, the oxygen always 

 sustains to the hydrogen the proportion of 8, by weight, the hydrogen 

 being 1, and when they are in the gaseous state, those proportions 

 will be found to correspond to 2 volumes of hydrogen and 1 of oxy- 

 gen ; their specific gravities being in the proportion of 1 hydrogen to 

 16 oxygen, it of course requires a double volume of hydrogen to sus- 

 tain the proportion by weight of 1 to 8. 



(pp>) In order that numbers may express correctly the combining 

 power of bodies, they must refer to a common unit. -Oxygen and hy- 

 drogen are the bodies which have been selected for this purpose : dif- 

 ferent philosophers have adopted, some the one and some the other 5 

 but there is in my view a decided advantage in adopting hydrogen, 

 and in expressing its lowest combining proportion by 1 We thus 

 avoid fractional expressions, for it would appear from the researches 

 of Prout and others, that the combining powers of all bodies may be 

 expressed by numbers which are multiples or reduplications of that 

 which expresses the combining power of hydrogen. We go upon 

 the supposition that hydrogen enters into combination with oxygen to 

 form water, in a smaller proportion than it enters into the constitution 

 of any other body ; and also that there is no body whatever that en- 

 ters into combination in so small a proportion as hydrogen* We 

 have, it is true, only negative evidence in support of either of these 

 propositions, although the presumption that they are true amounts al- 

 most to certainty. But should it be hereafter discovered that hydro- 

 gen enters into some combination in a less proportion than it exists in 

 water ; or that some other element enters into combination in a pro- 

 portion still smaller than any known proportion of hydrogen ; even 



* This most remarkable fact evidently depends upon the original constitution of 

 things ; and is as truly a law of the physical universe, as that its gravitation is direct- 

 ly as the quantity of matter, and inversely as the square of the distance. 



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