1880.] Dr. Ramsay. On the Critical State of Gases. 325 



As soon as a meniscus was visible, the liquid boiled so violently as to 

 give the appearance of a denser mist. 



4. A tube of the shape shown was filled to the mark with methyl 

 formate and heated in an inclined position, the portion containing the 

 liquid being the lower. The liquid, as usual, expanded, and at the 

 moment when the meniscus disappeared it nearly filled the lower half. 

 The Bunsen's burner below the copper heater was then withdrawn, and 

 on cooling, the liquid all condensed in the lower half. 



5. The last experiment was varied by tilting the heater after the 

 meniscus of the liquid had disappeared, so that that half which had 

 contained the liquid was uppermost. On cooling, the liquid condensed 

 in the upper half of the tube. 



G. The experiment was again varied by keeping the tube at a tem- 

 perature a few degrees above the point where the meniscus vanished, 

 for half an hour. On cooling, an almost equal quantity had condensed 

 in each division of the tube. (During Experiments 4, 5, and 6 great 

 care must be taken to keep the heater from draughts of cold air, 

 otherwise unequal cooling results and distillation takes place.) 



7. It was noticed that that half of the tube containing liquid, 

 after the meniscus had vanished, appeared full, while the other 

 half of the tube seemed to be empty. The refractive indices of the 

 flui,d contained in the tubes were therefore different. The por- 

 tion of the tube containing liquid was shown to be a more powerful 

 cylindrical lens than the empty portion, for on focussing a spot behind 

 the tube with a microscope, the focus was shorter when the portion 

 which had contained liquid was placed between the microscope and 

 the spot than when the portion appearing empty was interposed. 



8. From experiments on the expansion of liquids above their boiling- 

 points, of which numerical details shall be given on a future occasion, 

 it appears probable that the specific gravity of the hot liquid, at the 

 temperature at which the meniscus vanishes, is identical with that of 

 the compressed gas evolved from the liquid. This has also been 

 noticed by Ansdell in two cases, viz., hydrogen chloride and acetylene. 



9. I am inclined to believe, from observations on the expansions of 

 liquids at high temperatures, although they were not expressly directed 

 to elucidate this point, that liquids above the temperatures at which 

 their menisci vanish are not uniformly compressible. Bat as further 

 experiments are necessary, I mention this opinion with some hesita- 

 tion. 



From these observations I would draw the following inferences : — 



