1014 Transactions of the American Institute. 



snicall changes of refrangibilitj are not required, since in tins instru- 

 ment these changes may be measured by the changes in the position 

 of objects completely similar in kind. 



The Continuity or the Gaseous and Liquid States of Matter. 

 In a paper on this subject, read before tiie Royal Society of London, 

 Dr. Thomas Andrews gives an account of various experiments made 

 by him, during a series of years, upon gases, especially describing 

 the combined effects of heat and pressure upon carbonic acid gas at 

 temperatures varying from thirteen degrees Centigrade to forty-eight 

 degrees Centigrade, and at pressures ranging from forty-eight to 109 

 atmospheres. The temperature at which this gas ceases to liquify 

 l)y pressure he finds to be 30.92 degrees Centigrade, or eiglity-eight 

 degrees Fahrenheit, and tliis he calls the critical point. Although 

 liquefaction does not occur at temperatures a little above this point, 

 a very great change of density is produced by slight alterations of 

 pressure, and the flickering movements are conspicuous M'hich render 

 fainter the surface of demarcation between the liquids and gas. As 

 the direct result of his experiments, he concludes that the gaseous 

 and liquid states are only widely separated forms of the same condi- 

 tion of matter, and may be made to pass into one another by a 

 series of gradation so gentle that the passage shall nowhere present 

 any interruption or breach of continuity. From carbonic acid as a 

 perfect gas to carbonic acid as a perfect liquid the transition may be 

 accomplished b}^ a continuous process, and the gas and liquid are 

 only distant stages of a long series of continuous changes. Under 

 certain conditions of temperature and pressure, carbonic acid finds 

 itself, it is true, in a state of instability, and suddenly passes, without 

 change of pressure or temperature, but with the evolution of heat, to 

 tlie condition which, by the continuous process, can only be reached 

 by a long and circuitous route. The author discusses the question 

 as to what is tlie condition or state of carbonic acid when it passes at 

 temperatures above thirty-one degrees Centigrade from the ordinary 

 gaseous state down to the volume of the liquid, without giving 

 evidence during tlie process of the occurence of liquefaction, and 

 arrives at the conclusion that the answer to this question is to be 

 found in the intimate relations which subsist between the gaseous 

 and liquid states of matter. In tlie abrupt change Avhich occurs wlien 

 the gases are compressed to a certain volume at temperatures below 

 the ci'itical point, molecular forces are brought into play wliich pro- 



