1880.] State of Fluids at their Critical Temperatures. 479 



comes a gas. But a question arises. To observe this disappearance of 

 the cohesion of a liquid, it is requisite that it should have a free surface, 

 and this free surface has till now only been obtained by arranging the 

 pressure that a portion of the fluid is in the gaseous state, and this 

 only occurs at one pressure. Now, when the temperature of a liquid 

 is raised while it is retained under very great pressure, so that it never 

 has a free surface, but is always retained filling the vessel, does the 

 liquid still lose its cohesion, and become a gas at the same tempera- 

 ture ; or, as the pressure is increased, does the temperature at which 

 the cohesion of the liquid is overcome, also rise ? In the former case, 

 the limit of the liquid state would be an isotherm, in the latter, a con- 

 tinuation of the boiling line. To determine which is the object of the 

 work here described. 



With proper precautions, the loss of cohesion or capillarity can be 

 noticed very accurately, and the level of the liquid in a fine capillary 

 tube, seen to coincide with the plane surface of the liquid just before 

 the final disappearance of the line of demarcation. One of the precau- 

 tions to be taken is to obtain equable temperature, and while in my earlier 

 experiments, I used a double air-bath, and considered this sufficient to 

 obtain good results, I subsequently found that by the use of a triple 

 bath of copper, every trace of irregularity of temperature disappeared, 

 and I obtained results in which the line of division was admirably 

 clear and sharp, and never became broad and hazy as in ordinary ex- 

 periments. Another precaution to be taken is to have pure liquids, 

 and this at first sight might appear to be an easy matter, but I find 

 that in transferring a portion of a pure liquid to a tube, the momentary 

 exposure to air, especially in the vicinity of the hands, hydrates the 

 liquid sufficiently to render the line of demarcation rounded, and show 

 a slightly greater refractive power in the lower part of the tube after 

 the critical point has been passed. In the case of liquefied gases, such 

 as carbon dioxide, ammonia, sulphur dioxide, and nitrous oxide, which 

 are easily dried, the line is beautifully sharp, and the disappearing 

 point easily noted. Alcohol cohobated over caustic lime for a week 

 and transferred to a tube without contact with air, shows the dis- 

 appearance of the line with great sharpness, and immediately after 

 no difference in refractive power can be detected between the upper 

 and lower portions. The least trace of moisture is sufficient to show 

 such a difference. Whenever I notice any difference between the 

 upper and lower portions after passing the critical point, I attribute it 

 to moisture or other impurity, as careful treatment always removes 

 the difference in density. In many organic liquids there is always a 

 difference at the critical point, and sometimes before reaching this 

 temperature, they form several layers, each having a different 

 critical point as they seem to give rise on heating to new compounds, 

 or form polymeric compounds having different critical points. Besides, 



