M. Regnault on the Elastic Force of Vapours, 275 



a difficulty in knowing the precise time for a revolution of the 

 sun on his axis, the period varying (I presume as measured by 

 the spots on his surface) as much as two days in certain regions 

 or latitudes, in excess of the equatorial. Still the influence and 

 position of the planet Mercury may have considerable magnetic 

 effects. 



It must also be borne in mind, while attempting to connect 

 planetary influences through mass-attraction, with the secular 

 returns of &pot-maximum and minimum, that those periods are 

 not, as far as they have hitherto been examined, precisely regular 

 or fixed; the average, taken from before 1800 to 1860, gives, 

 according to Wolf, a period of 11*1 years; but from the years 

 1826 to 1850, the best observations made by Schwabe show that 

 the period was then a decennial one, with maximam 1828, 1837, 

 and 1848, and minima in 1833 and 1843; and Sabine states 

 " that during the same period the solar magnetic variations show 

 likewise a decennial period strictly coincident with the solar spots." 



XXXIV. On the Elastic Force of Vapours. By M. Regnault. 



[The important researches of M. Regnault on the Elastic 

 Force of Vapours under various conditions, abstracts of which 

 have been given in the Numbers of this Magazine for October 

 1854 and January 1855, have just appeared in full in the 

 twenty-sixth volume of the Memoires de I' Academic. In calling 

 attention to their publication, in the Comptes Rendus for June 11, 

 1860, the author at the same time communicates an account of 

 some further developments which he has given to a part of this 

 research — that which treats of the tension of saturated vapours 

 in vacuo, a translation of which we proceed to lay before our 

 readers. — Eds.] 



The various apparatus which I have used in these researches 

 are described in the original memoir. I will merely remark 

 that they refer to two different methods. 



The first, which I call the statical method, consists in deter- 

 mining the pressure which is equal to the elastic force of a vapour 

 at rest disengaged from a liquid in excess. In the second method, 

 which is called the dynamic method, the vapour is always in motion, 

 and a determination is made of the temperature of the vapour 

 continually emitted by the liquid boiling under different pressures. 



Both methods give the same results, 



1. When the liquid is quite homogeneous. This is not the 

 case when it is impure ; the presence of the smallest quantity of 

 a volatile foreign body is immediately seen in the non-superpo- 

 sition of the two graphic curves which belong to one or the other 

 of these methods. 



