1880.] On the Critical Point of Mixed Vapours. 541 



commenced to condense on the sides before the trichloride of phos- 

 phorus came in sight, and when the latter was visible, a slight in- 

 distinct layer of fluid was seen on the surface, distinguishable by 

 the different refractive index of the two liquids, there being no sharp 

 line of demarcation. On standing a few minutes, the liquids became 

 quite homogeneous. On increasing the pressure, more carbonic acid 

 was condensed, forming a more or less distinct layer on the surface of 

 the trichloride of phosphorus. This, however, rapidly disappeared on 

 standing. On releasing the pressure, the carbonic acid boiled first on 

 the surface, but afterwards through the liquid, the trichloride of phos- 

 phorus at the same time falling in heavy stria? to the bottom. 



At 23° C. the carbonic acid appeared to liquefy at 46*91 atmo- 

 spheres, exactly the same appearances taking place as at 16° C. 



At 30° C. the carbonic acid liquefied at 49*94 atmospheres, forming 

 a rather more distinct layer, and not mixing so readily. On increasing 

 the pressure to 90 atmospheres, the surface of the carbonic acid dis- 

 appeared, it being near its critical point, the top part of the tube 

 being filled with a homogeneous mass. The trichloride of phosphorus- 

 could also not be distinguished on the surface of the mercury, its 

 upper surface being entirely mixed up with the carbonic acid, the 

 whole space above the mercury forming one homogeneous mass. 



On releasing the pressure, a cloud first appeared, and then the sur- 

 face of the carbonic acid became visible ; it boiled away first from the 

 surface, and afterwards through the trichloride of phosphorus. 



At 33°C, liquid carbonic acid appeared at 50*84 atmospheres. 

 At 40 „ „ „ 56*88 



At 50 „ „ „ '66*53 



At all the temperatures above 30°C, the appearances were the same, 

 except that as the temperature increased, the quantity of fluid liquefied 

 diminished, and it took a greater pressure to make the surface of the 

 trichloride of phosphorus disappear. 



The following are pressures taken with a smaller quantity of car- 

 bonic acid and trichloride in the tube, and were read off when the sur- 

 face of the trichloride was first agitated, thus showing that the- 

 carbonic acid had commenced to condense. 



At 10 -5 C 22-70 atmospheres. 



„ 16-5 „ 24*70 



„ 22 -8 „ 32 *18 



„ 30 *0 „ 33 *88 



„ 40-0 „ 36-36 



„ 50 -0 „ 49 -67 



„ 70 *0 „ 76-61 



