202 



bodies are thus related to one another, it is the proto- compounds, or 

 those containing single proportions, which are decomposable, and that 

 the per -compounds are not so. 



The second section contains an account of a new instrument devised 

 by the author, for exactly measuring electric currents, and which he 

 terms the volta- electrometer. The current to be measured is made 

 to pass through water acidulated by sulphuric acid, and the gases 

 evolved by its decomposition are collected and measured, thereby 

 giving at once an expression of the quantity of electricity which has 

 passed. The principle on which this conclusion is founded is the 

 new law discovered by the author, " that the decomposing action of 

 any current of electricity is constant for a constant quantity of electri- 

 city. ,f The accuracy of this law was put to the test in every possible 

 way, with regard to the decomposition of water, by making the same 

 current pass in succession through two or more portions of water, 

 under very different circumstances : but whatever were the variations 

 made, whether by altering the size of the poles or electrodes, by in- 

 creasing or lessening the intensity of the current or the strength of 

 the solution, by varying its temperature or the mutual distance between 

 the poles, or by introducing any other change in the circumstances 

 ot the experiment, still the effect was found to be the same j and a 

 given quantity of electricity, whether passed in one or in many por- 

 tions, invariably decomposed the same quantity of water. No doubt, 

 therefore, remains as to the truth of the principle on which the volta- 

 electrometer acts : but with regard to the practical application of the 

 principle, several forms of the instrument are described by the author, 

 and the mode of employing them, either as the measurers of absolute 

 quantities, or as standards of comparison, are fully pointed out. 



In the third section of the paper, the primary or secondary charac- 

 ter of the bodies evolved at the electrodes is discussed. It is shown 

 that they are secondary in a far greater number of cases than has 

 usually been imagined ; and that laws have been deduced with regard 

 to the ultimate places of substances, from the appearance of the se- 

 condary products 5 so that certain conclusions, true in themselves, 

 have hitherto been obtained by erroneous reasoning, since the facts 

 which were supposed to support them have, in truth, no direct relation 

 with those conclusions. The methods of distinguishing primary and 

 secondary results from each other are explained, and the importance 

 of this distinction towards the establishment of the law of definite 

 electro-chemical action is insisted upon by the author. 



The fourth section is entitled, " On the definite Nature and Extent 

 of Electro chemical Decomposition," and is considered by the author 

 as by far the most important of this or indeed of the whole series of 

 investigations of which he has now presented the results to the Royal 

 Society. He adverts to the previous occasions on which he has al- 

 ready announced, more or less distinctly, this law of chemical action ; 

 and also to the instrument just explained as one of the examples of the 

 principle about to be developed. He next refers to experiments de- 

 scribed in another part, in which primary and secondary results are di- 

 stinguished as establishing the same principle with regard to muriatic 



