38 Royal Society. 



It is now, however, universally agreed, that all abstractions and 

 generalizations are mere creatures of the reasoning faculty, existing 

 nowhere but in the mind contemplating them. Such, in algebra, 

 are the supposed even roots of a real quantity, taken in the scale 

 opposite to that which has given the universal antecedent : the 

 sign indicating the extraction impossible to be performed, veils 

 the real quantity, and renders it of no actual value until the sign 

 is taken away by an involution, the reverse of the supposed opera- 

 tion which the sign represents ; although the quantity itself is, in 

 the mean time, by its arbitrary essence, made applicable to all the 

 purposes for which real quantities are used, in every kind of for- 

 mula. 



Several illustrations of these views of the nature of imaginary 

 quantities occurring in logarithmic formulae, and series expressing 

 circular arcs, are given by the author. By considering all quantity 

 as affirmative per se, and admitting plus and minus merely as con- 

 nective terms, we thus succeed in banishing mystery and para- 

 dox from the science most powerful in eliciting truth, and where they 

 ought least to find a place. 



Nov. 25. A paper was read, entitled, " On a simple electro- 

 chemical method of ascertaining the presence of different metals ; 

 applied to detect minute quantities of metallic poisons." By Ed- 

 mund Davy, Esq. F.R.S., M.R.I. A., and Professor of Chemistry to 

 the Royal Dublin Society. 



The Voltaic arrangement employed by the author consisted 

 merely of small slips of different metals, generally zinc and platina, 

 placed in contact and forming a galvanic circuit with the inter- 

 posed fluid suspected to contain the poisonous metal ; in which case, 

 as was formerly shown by Sir H. Davy in his Bakerian lecture, the 

 metal held in solution was deposited in the form of crystals, on the 

 negative surface. The zinc was usually employed in the form of 

 foil ; the platina was, in some cases, a small crucible, or a spatula, 

 but more frequently platina foil was used. It is generally necessary 

 to mix a few drops of acid with the metallic compounds that are 

 subjected to this test, and that are placed in contact with the pla- 

 tina : on applying the zinc foil, the platina will soon become coated 

 with the reduced metal. 



The author then enters into the detail of his experiments on the 

 efficacy of his method in the detection of arsenic, mercury, lead and 

 copper, in their different states of oxidation and saline combina- 

 tions ; and of the precautions necessary to be observed in the case 

 of each metal. He was enabled to detect the presence of arsenic, 

 by the exhibition of its characteristic properties, when only the 

 500dth part of a grain of that metal was deposited on the platina; 

 and in some instances could appreciate even the 2500dth part of a 

 grain, by the application of appropriate tests. 



The author next ascertained that the electro- chemical method is 

 competent to the detection of very minute quantities of the differ- 

 ent metals, when their compounds are mixed with various vegeta- 

 ble and animal substances. Thus, the presence of arsenic would 

 readily be discovered when mixed with all the ordinary articles of 



diet, 



