176 



HEAT. 



Assuming that Boyle's law holds, the weight of a volume of gas at 0* 

 and 760 mm. equal to that of the globe is 



Wx760 



FIG. 100. Vapour-Density Bulb : 

 Dumas' Method. 



Various corrections are of course needed, and we may refer the reader 



to Regnault's experiments, of which 

 a short account is given in Jamin, 

 Cours de Physique, torn, ii., or to the 

 series of papers describing the ex- 

 periments which led Lord Rayleigh 

 to the discovery of argon (Proc. R.S., 

 Feb. 1888, Feb. 1892, March 1893; 

 Phil. Trans., A., 1895, p. 187). 



Of course, this method is not 

 applicable to substances which are 

 liquid at ordinai-y temperatures and 

 pressures. In their case, the weight 

 of a known volume of the vapour 



must be determined at a known pressure, and at such a temperature 



that there is no condensation at that pressure, and the ratio of 



this to the weight of the same 



volume of air at the same tem- 



perature and pressure is termed 



the " Vapour-Density." 



Various arrangements have 



been adopted for the purpose. 



In Dumas' method, a bulb (Fig. 



100) ending in a fine open tube, 



is first filled with dry air and 



weighed, the temperature and 



pressure being observed. Some 



of the substance is then put into 



the bulb in the liquid state, and 



the bulb is immersed in a water- 



or oil-bath, the temperature of 



which is increased somewhat 



above the boiling-point of the 



liquid, the open tube projecting 



above the liquid in the bath. 



The liquid boils, and, if a suf- 



ficient quantity is present, its 



vapour drives out the air. When 



all the liquid has evaporated, and FIG. 101. Vapour-Density Determination by 



the vapour is at the tempera- Gay Lussac and Hoffmann's Method. 



ture of the bath, the end of the 



fine tube is sealed. The bulb is now taken out, and when cool is 



weighed again. The point of the tube is now broker under mercury, 



the vapour condenses, and the bulb is practically filled with mercury. 



From its weight so filled, its volume can be determined. From this we 



may find the weight of the air filling it in the first weighing, and thence 



