Notices respecting New Books. 4^ 



Mariotte's law. The Hflh series of experiments was for tlie purpose 

 of seeking the dilatation of the gas in a direct manner ; for this it 

 would be requisite to enclose it in a very elastic envelope, so that it 

 might dilate freely without change of the elastic force, and that the 

 increase of volume it would undergo might be measured with pre- 

 cision, the whole mass remaining at the same temperature. It was 

 difficult to realize these conditions practically, but M. Regnault has 

 approached them, by following a method analogous to that already 

 employed by M. Pouillet for his air pyrometer, and which consists 

 in having a kind of reservoir filled with mercury, communicating 

 with the balloon containing the gas experimented on, and causing the 

 mercury necessary to give place to the gas, to flow in such a manner 

 that it may obey its dilatation, without change of elastic force. 



It is difficult to conceive all the precautions which are required 

 both in this and the other methods, to ascertain exactly the capacity 

 of the balloons and tubes which contain the gas, to ensure thorough 

 desiccation of the air, to bring to one given temperature all the parts 

 of the apparatus, and in particular the columns of mercury, which, 

 by their height, serve to measure the variations of the elastic force, 

 or those of the volume, and to determine with the necessary precision 

 the heights of the columns of mercury. The very clear detail of 

 his experiments must be read in M. Regnault's work itself, the de- 

 scription of all his apparatus must be followed on the plates which 

 accompany his memoirs, to understand the nature of the practical 

 difficulties which are presented by researches of this kind, and the 

 manner of securing himself from all the causes of error, so nume- 

 rous, which the experimenter necessarily meets on his way. One 

 of the most difficult operations, until the habit has been acquired, is 

 the measurement of the height of the mercury raised. M. Regnault 

 employed for this purpose the cathetometer of Gambey, which, by 

 its vernier, gives an immediate reading to the fiftieth of a milli- 

 metre. This cathetometer is a divided vertical scale, fixed on a solid 

 point, and bearing a traveller furnished with a perfectly horizontal 

 lens. The operator looks with the lens toward the level of the mer- 

 cury raised in the tube, then the lens is lowered, and he looks at the 

 upper end of a screw, the lower end of which just touches the sur- 

 face of the mercury in the vessel in which the tube is plunged ; by 

 adding to the difference of level thus obtained, the distance between 

 the two points of the screw which had been previously measured 

 with the same instrument, the total height of the mercury raised is 

 obtained. 



The five series of experiments gave the following means for the 

 volume of gas at 100°, its volume at 0° being 1 : — 



First series 1*36623 



Second series 1-36633 



Third series 1-36679 



Fourth series 1*36650 



Fifth series 1-36706 



The first four methods, although different, give nearly the same 



