132 ANNUAL UF SCIENTIFIC DISCOVERY. 



a state of instability, and suddenly passes with the evolution of 

 heat, and without the application of additional pressure or change 

 of temperature, to the volume which by the continuous process 

 can only be reached through a long and circuitous route. In the 



/ d? C_7 



abrupt change which here occurs, a marked difference is exhib- 

 ited, while the process is going on, in the optical and other physi- 

 cal properties of the carbonic acid which has collapsed into the 

 smaller volume, and of the carbonic acid not yet altered. There 

 is no difficulty here, therefore, in distinguishing between the liquid 

 and the gas. But in other cases the distinction cannot be made ; 

 and under many of the conditions I have described it would be 

 vain to attempt to assign carbonic acid to the liquid rather than 

 the gaseous state. Carbonic acid, at the temperature of 35.5, 

 and under a pressure of 180 atmospheres, is reduced to 0.480 of 

 the volume it occupied under a pressure of one atmosphere ; but 

 if any one ask whether it is now in the gaseous or liquid state, 

 the question does not, I believe, admit of a positive reply. Car- 

 bonic acid at 35.5, and under 108 atmospheres of pressure, stands 

 nearly midway between the gas and the liquid ; and we have no 

 valid grounds for assigning it to the one form of matter more than 

 to the other. The same observation would apply with even 

 greater force to the state in which carbonic acid exists at higher 

 temperatures and under greater pressures than those just men- 

 tioned. 



" In the foregoing observations I have avoided all reference to 

 the molecular forces brought into play in these experiments. The 

 resistance of liquids and gases to external pressure tending to 

 produce a diminution of volume proves the existence of an inter- 

 nal force of an expansive or resisting character. On the other 

 hand, the sudden diminution of volume, without the addition of 

 external pressure, which occurs when a gas is compressed at any 

 temperature below the critical point to the volume at which lique- 

 faction begins, can scarcely be explained without assuming that 

 a molecular force of great attractive power comes into operation, 

 and overcomes the resistance to diminution of volume which com- 

 monly requires the application of external force. When the pas- 

 sage from the gaseous to the liquid state is effected by the con- 

 tinuous process described in the foregoing pages, these molecular 

 forces are so modified as to be unable at any stage of the process 

 to overcome alone the resistance of the fluid to change of vol- 

 ume. 



" The properties described in this communication as exhibited 

 by carbonic acid, are not peculiar to it, but are generally true of 

 all bodies which can be obtained as gases or liquids. Nitrous 

 oxide, hydrochloric acid, ammonia, sulphuric ether, and sulphuret 

 of carbon, all exhibited, at fixed pressures and temperatures, 

 critical points, and rapid changes of volume with flickering move- 

 ments, when the temperature or pressure was changed in the 

 neighborhood of those points. The critical points of some of 

 these bodies were above 100 ; and in order to make the obser- 

 vations it was necessary to bend the capillary tube before the 



