NATURAL PHILOSOPHY. 



205 



circuit thus formed is introduced a galvanometer, g, consisting of a magnetic 

 needle, which swings freely within a coil of covered copper wire : the passage 



of an electric current through the coil 

 being, as is well known, rendered man- 

 ifest by the deflection of the needle. 

 Let the drop, d, be rendered a good 

 conductor of electricity, by slightly" 

 acidulating it ; if it were in contact 

 with the basin, the circuit would at 

 no place be interrupted ; the current 

 would pass without hindrance from n 

 to the basin, thence through the drop 

 to the platinum wire, e, and thence 

 through the galvanometer to the 

 other pole of the battery. In its 

 h, passage it would deflect the needle 

 of the galvanometer, and thus give 

 evidence of its presence. It is, how- 

 ever, found that when the basin is 



71 



111 



\J 



heated, and the drop has assumed the spheroidal state, no current passes ; 

 and this certainly indicates the existence of an interval which interrupts the 

 circuit between the basin and the drop. Let the lamp which heats the 

 basin be now removed ; after a time the drop sinks, comes into contact with 

 the basin, and at that instant the needle of the galvanometer flies aside, thus 

 demonstrating die passage of the current." 



INFLUENCE OF TEMPERATURE ON CAPILLARY ATTRACTION. 



The influence of temperature on capillarity, considered null by some au- 

 thors, is still real. Lalande was the first to announce it, 1768 ; he showed 

 that water did not stand as high when it was hot, or when the tube had been 

 heated before making the experiment. Laplace and Poisson deduced from 

 it that the height had some relation to the density of the liquid. Many em- 

 inent physicists have treated of this question, and from their results a wide 

 divergence has been brought about between mathematical theory and ex- 

 periment, and a complete separation of the phenomena of ascension and de- 

 pression. All these questions have been taken up from their foundation and 

 definitely resolved by the laborious researches of M. Wolf, Professor of 

 Physics at the Lyceum of Strasburg, which have been continued through 

 several years. The following are the facts arrived at : 



1. The elevation of the same liquid in capillary tubes depends, other 

 things equal, on the nature of the tube. 



2. In the same tube, at different temperatures, the height to which a liquid 

 rises is in the compound proportion of the density and the curvature of the 

 meniscus ; this diminishes as the temperature increases, and becomes null 

 at some specific temperature beyond which, the action is the reverse. The 

 law of variation of pressure with the temperature for liquids which do not 

 wet the glass, thus connects with the law of diminution of capillary eleva- 



18 



