the same time the part was filled with solid carbonic acid. It melts at the temperature of -70° or -72° Fahr., and the solid carbonic acid is heavier than the fluid bathing it. The solid or liquid carbonic acid at this temperature has a pressure of 5.33 atmospheres nearly. Hence it is easy to understand the readiness with which liquid carbonic acid, when allowed to escape into the air, exerting only a pressure of one atmosphere, freezes a part of itself by the evaporation of another part.
Thilorier gives -100° C. or -14.8° Fahr. as the temperature at which carbonic acid becomes solid. This however is rather the temperature to which solid carbonic acid can sink by further evaporation in the air, and is a temperature belonging to a pressure not only lower than that of 5.33 atmospheres, but even much below that of one atmosphere. This cooling effect to temperatures below the boiling-point often appears. A bath of carbonic acid and ether exposed to the air will cool a tube containing condensed solid carbonic acid, until the pressure within the tube is less than one atmosphere; yet, if the same bath be covered up so as to have the pressure of one atmosphere of carbonic acid vapour over it, then the temperature is such as to produce a pressure of 2.5 atmospheres by the vapour of the solid carbonic acid within the tube.
The estimates of the pressure of carbonic acid vapour are sadly at variance; thus, Thilorier[1] says it has a pressure of 26 atmospheres at -4° Fahr., whilst Addams[2] says that for that pressure it requires a temperature of 30°. Addams gives the pressure about 27½ atmospheres at 32°, but Thilorier and myself[3] give it as 36 atmospheres at the same temperature. At 50° Brunel[4] estimates the pressure as 60 atmospheres, whilst Addams makes it only 34-'67 atmospheres. At 86° Thilorier finds the pressure to be 73 atmospheres; at 4° more, or 90°, Brunel makes it 120 atmospheres; and at 10° more, or 100°, Addams makes it less than Thilorier at 86°, and only 62°32 atmospheres; even at 150° the pressure with him is not quite 100 atmospheres.
I am inclined to think that at about 90° Cagniard de la Tour's state comes on with carbonic acid. From Thilorier's data we may obtain the specific gravity of the liquid and the
