408 Intelligence and Miscellaneous Articles. 



and the importance of this question will be especially apparent if: 

 we observe that in the best treatises superabundant data are taken 

 from observation, even for constructing the simplest theory of all 

 (that of gases). We then arrived at a result which can in brief be 

 enunciated thus : — To know all the isothermal lines of a body, and 

 one of its adiabatic lines, is sufficient. 



The law which forms the object of the present investigation con- 

 ducts to the following much more satisfactory and quite unexpected 

 result : — In order to know all the isothermal lines and all the adiabatic 

 lines of a body, and consequently to be able to study it completely, 

 it is necessary and sufficient to know two of its isothermal lines and 

 one only of its adiabatic lines. 



In physical terms, one may say that it is sufficient to observe : — 

 1st, the dilatation of a body under two different pressures, or, more 

 generally, for two series of states answering to two curves arbitra- 

 rily traced in the plane of the (pv)'a (which is equivalent to saying 

 that the oo 2 observations, of which we spoke at the outset of our 

 previous communication, are replaced by two simple infinities of 

 observations) ; 2ndly, one of the specific hea ts, or one particular 

 pressure only, or, more generally, for a single series of states of the 

 body corresponding to a curve arbitrarily traced in the plane. 



If w-e admit, with MM. Clausius and Hirn, that the thermal 

 capacity of every substance is a constant, this second series of ob- 

 servations reduces itself to a single observation. — Comjptes Rendus 

 de VAcademie des Sciences, Sept. 30, 1878, t. lxxxvii. pp. 488-491. 



THE SONOROUS VOLTAMETER. BY THOMAS A. EDISON, PH.D. 



The sonorous or bubble voltameter consists of an electrolytic cell 

 with two electrodes — one in free contact with a standard decompo- 

 sable solution, and the other completely insulated by vulcanized 

 rubber except two small apertures, one of which gives the solution 

 free access to the insulated electrode, and the other allows the 

 escape of bubbles of hydrogen as they are evolved by electrolysis. 

 With a given current and a given resistance a bubble is obtained 

 each second, which is seen at the moment of rising, and which at 

 the same time gives a sound when it reaches the air. The resist- 

 ance may be reduced so as to give one bubble in one, five, ten, or 

 fifty seconds, or in as many hours. I have compared this instru- 

 ment with the ordinary voltameter, and find it much more accurate. 

 By the use of a very small insulated electrode and but one aperture, 

 through which both the gas and water current must pass, great in- 

 crease of resistance takes place at the moment when the bubble is 

 forming ; and just before it rises, a Sounder magnet included within 

 the battery-circuit opens, closing again when the bubble escapes, 

 thus allowing by means of a Morse register the time of each bubble 

 to be recorded automatically. This apparatus, when properly made, 

 will be found very reliable and useful in some kinds of work, such 

 as measuring the electromotive force of batteries &c. By shunting 

 the voltameter and using a recorder it becomes a measurer, not only 

 of the current passing at the time, but also of that which has passed 

 through a circuit from any source during a given interval. — Silli- 

 man's American Journal, November 1878. 



