2SO THE POPULAR SCIENCE MONTHLY. 



perature (21° C, or 70° Fahr.), he found that the same result was pro- 

 duced, but more slowly ; and it seemed to be heralded in advance by 

 a more rapid diminution in volume previous to the beginning of the 

 change, which continued after the jDrocess had been accomplished ; as 

 if an anticipatory preparation for the liquid state were going on pre- 

 vious to the completion of the change. Performing the experiment 

 again at 32° C. (90° Fahr.), the anticipatory preparation and the after- 

 continuation of the contraction were more marked, and, instead of a 

 separate and distinct liquid, wavy and mobile striae were perceived on 

 the sides of the vessel as the only signs of a change of state which 

 had not yet been effected. At temperatures above 32° C. (90° Fahr,), 

 there were neither strise nor liquefaction, but there seemed to be a 

 suggestion of them, for, under a particular degree of pressure, the 

 density of the gas was augmented, and its volume diminished at an 

 increasing rate. The temperature of 32° C. (90° Fahr.) is, then, a 

 limit, marking a division between the temperatures which permit and 

 those which prevent liquefaction ; it is the critical point, at which is 

 defined the separation, for carbonic acid, between two very distinct 

 states of matter. Below this point, the particular matter may assume 

 the aspect of a liquid ; above it, the gas can not change its ap-pesir- 

 ance, but enters into the opposite constitution from that of a liquid. 



Generally, a liquid has considerably greater density than its vapor. 

 But, if a vessel containing both is heated, the liquid experiences a dila- 

 tation which is gradually augmented till it equals and even exceeds 

 that of the gas ; whence, of course, an equal volume of the liquid will 

 weigh less and less. On the other hand, a constantly larger quantity 

 of vapor is formed, which accumulates above the liquid and becomes 

 heavier and heavier. Now, if the density of the vapor increases, and 

 that of the liquid diminishes, they will reach a point, under a suitable 

 temperature, when they will be the same. There will then be no rea- 

 son for the liquid to sink or the vapor to rise, or for the existence of 

 any line of separation between them, and they will be mixed and con- 

 founded. They will no longer be distinguishable by their heat of con- 

 stitution. It is true that, in passing into the state of a vapor, a liquid 

 absorbs a great deal of latent heat, but that is employed in scattering 

 the molecules and keeping them at a distance ; and there will be none 

 of it if the distance does not increase. We are then, at this stage of 

 our experiments, in the presence of a critical point, at which we do not 

 know whether the matter is liquid or gaseous ; for, in either condition, 

 it has the same density, the same heat of constitution, and the same 

 properties. It is a new state, the r/aso-liquid state. An experiment 

 of Cagniard-Latour re-enforced this explanation of the phenomena. 

 Heating ether iu closed vessels to high temperatures, he brought it to 

 a point where the liquid could be made wholly to disappear, or to be 

 suddenly reformed on the slightest elevation or the slightest depres- 

 sion of temperature, accordingly as it was raised just above or cooled 



