346 Dr. Macneveti's Exposition of [Nov. 



cise quantity of pure potash, or carbonate of potash, required for 

 the precipitation of the copper. Lastly, zinc or iron may be used for 

 the same purpose, and then it will, perhaps, be desirable to know 

 how much sulphate of zinc or sulphate of iron remains in 

 solution. 



Were we to resolve many such questions, we should find it to 

 be extremely fatiguing, and to waste a great deal of time ; 

 whereas a considerable deal of both is saved to the experimental 

 chemist, whenever he can have recourse to a prior analysis that 

 is executed with all due care and accuracy. 



The synoptic scale of chemical equivalents resolves all these 

 questions by bare inspection, as relates to many of the salts con- 

 tained in the table. It not only gives the numerical proportions 

 on which the desired solution is calculated, but it expresses the 

 precise weights of the different constituents of a given quantity 

 of the salt under examination, no less than the quantity of the 

 reagents necessary for its analysis, and that of the precipitates 

 which each of them throws down. Such a scale could not be 

 formed without a previous determination of the proportions in 

 which the different known bodies unite, and without their being 

 expressed in such terms that the same substance would be always 

 represented by the same number. 



This mode of designation is due to Richter ; he also was the 

 first who observed the law of constant proportions on which 

 alone these numerical representations can be founded. 



By the theory of Dalton, which is the one that best explains 

 the phenomena; chemical saturation results from the union of a 

 sino'le atom of each of the combining bodies ; and where one of 

 the constituents is in excess, then two or more atoms of this 

 combine with a single atom of the other. 



According to this view of the subject, when we count the 

 relative weights of the equivalents, Mr. Dalton conceives that 

 we count the united vi'eight of a given number of atoms, and 

 consequently include the proportion which exists between the 

 ultimate molecules of each of these substances. But, in the 

 case of two combinations of the same substances, since it is 

 difficult to determine which of them is the compound of one pair 

 of simple atoms, and since the decision of this question affects 

 only the theory, without being at all necessary to the formation 

 of a table intended for practical use. Dr. WoUaston has taken no 

 pains to make his numbers correspond with the hypothesis of 

 atoms. His object has been to render his table practical, and 

 he considers the doctrine of simple multiples, on which the 

 theory of atoms is founded, as only means of determining, by- 

 division, those quantities that do not precisely coincide with the 

 law of Richter. 



Dr. WoUaston took for the basis of his calculations and real 

 measure of comparison for determining equivalents, a determi- 

 nate quantity of carbonate of lime. This is a perfectly neutral 



