Intelligence and Miscellaneous Articles. 375 



machine, is perforated at that place, and more readily if the side on 

 which the drop lies be turned towards the positive electrode. 



A glass plate with bifilar suspension between the electrodes of a 

 Holtz machine is driven by the discharge toward the negative elec- 

 trode, and, indeed, with more force if the side turned to the positive 

 electrode is partially coated with stearine. 



Fixed points for an explanation of these facts are found by the 

 author in the assumption, once previously enunciated, and on that 

 occasion also advocated by Gr. Wiedemann, that the air molecules 

 in the spark's path are affected, in their certainly very energetic 

 motions, with a velocity- component directed from the positive to 

 the negative electrode, such as was originally attributed by Pliicker, 

 and after him by Eeitlinger, to the positive electricity itself. — 

 Kaiserliche Afcademie der Wissenschaften in Wien, math.-naturw. 

 Classe, March 6, 1879. 



ON THE PRESSURES EXERTED BY GALVANIC DEPOSITS. 

 BY M. BOUTY. 



If we take a thermometer with a cylindrical reservoir, render it 

 conductive by coating it with gold leaf or thinly silvering it, and 

 employ it as the negative electrode in the decomposition of a 

 copper-salt (for instance), the metallic deposit exerts a considerable 

 pressure upon the reservoir ; for the mercury rises in the stem the 

 more as the deposit is thicker. And to explain this ascent one can 

 neither invoke a local rise of temperature, which is insignificant — 

 nor an electrical action properly so called ; for the thermoinetrical 

 excess has no direct relation with the intensity of the current, and 

 persists integrally after the suppression of the latter. It depends 

 exclusively on the more or less perfectly metallic quantity of the 

 deposit, and will probably be capable of supplying the indirect 

 measure of it. Very crystalline or coarsely granular deposits 

 exert but a trifling compression. When we dissolve the metal 

 with an acid the thermometer becomes normal again. 



Professor Mills *, who discovered before I did, and without my 

 being aware of it when I commenced this investigation, the fact of 

 the contraction of thermometers, announced that copper, silver, 

 iron, and nickel contract, and that cadmium and zinc dilate the 

 reservoirs upon which they are applied. I have found that all 

 metals, including zinc, always act only by pressure ; but the pressure 

 is not necessarily normal, or the same at all points, and cannot 

 serve directly as a measure for the phenomenon. It is the result 

 of a change of volume undergone by the metal in being deposited. 

 I shall confine myself to the establishment of this point, reserving 

 for an ulterior Note all the peculiarities I have observed. 



Let us imagine that a cylinder M of external radius E, and 

 indefinite length becomes covered with a regular solid coat of which 

 th« external radius is B/. It undergoes a shrinkage, the amount 

 of which would be a fraction a of its internal volume if the cylinder 

 M offered no resistance ; but as it does resist, a normal pressure 

 * Proceedings of the Royal Society, vol. xxvi. p. 504. 



