Young— Vapour-Pressures, 6fc., of Tkirtij Pure Substances. 397 



(fig. 5). The tube must contain liquid at a known temperature from the end A 



to the etched mark B, any excess being 

 removed by tilting the tube and touch- 

 ing ^ with a piece of filter-paper. 

 When the tube is replaced in a vertical 

 position, some of the liquid flows back 

 into the little bulb ; and when expan- 

 sion takes place with rise of tempera- 

 ture, the liquid passes into the bulb. 

 A determination is first made with 

 water at 0° to find the capacity of the 

 tube, and afterwards with the liquid 

 either at 0° or with tlie tube immersed 

 in water of known temperature. Diffu- 

 sion of vapour through the air in the 

 narrow tubes is so slow that no appre- 

 ~*V^,^ ciable loss even of volatile liquids occurs 



Fig. 5. during the determination. The specific- 



gravity tube is suspended from the 



balance-beam by the looped platinum wire C. All weighings must be 



corrected for the buoyancy of the air. 



The density s = — , where m is the corrected weight of liquid, and v is the 



capacity of the tube from ^ to -B at the temperature of experiment. 



For higher temperatures the iron pressure-apparatus (fig. 3) was employed. 

 The weight of liquid in the tube was ascertained by reading the volume at a 

 number of temperatures between that of the room and 0°, and then at higher 

 temperatures at intervals of 10 degrees, the volumes in this and all similar 

 cases being corrected for the expansion of the glass. These volumes were 

 plotted against the temperatures ; and from the curve drawn through the 

 points readings were taken at the temperatures at wiiich the specific gravity 

 had previously been determined. The weight of liquid is the product of the 

 read volume into the specific gravity. The mean of the two (or more) values 

 was taken as correct. 



The volumes of a gram at the higher temperatures were then calculated 

 from the formula 



where V is the volume of a gram, v the read volume, and m the weight of 

 liquid. 



SCIENT. PROO. B.D.S., VOL. XII., NO. XXXI. 



3i 



