Properties of Matter in the Liquid and Gaseous States. 79 



or 75 feet, in length had to be employed. Tor pressures cor- 

 responding to 500 atmospheres, at which I have no difficulty in 

 working with my apparatus, a mercurial column of the enormous 

 height of 380 metres, or 1250 feet, would be required. Although 

 the mechanical difficulties in the construction of a long tube for 

 this purpose are perhaps not insuperable, it could only be mounted 

 in front of some rare mountain escarpment, where it would be 

 practically impossible to conduct a long series of delicate experi- 

 ments. About three years ago I had the honour of submitting 

 to the Council of the Society a proposal for constructing an ap- 

 paratus which would have enabled any pressure to be measured 

 by the successive additions of the pressure of a column of mer- 

 cury of a fixed length ; and working drawings of the apparatus 

 were prepared by Mr. J. Cumine, whose services I am glad to 

 have again this opportunity of acknowledging. An unexpected 

 difficulty, however, arose in consequence of the packing of the 

 screws (as I have already stated) not holding when the leather 

 was in contact with mercury instead of water, and the apparatus 

 was not constructed. For two years the problem appeared, if 

 not theoretically, to be practically impossible of solution; but I 

 am glad now to be able to announce to the Society that another 

 method, simpler in principle and free from the objections to 

 which I have referred, has lately suggested itself to me, by 

 means of which it will, I fully expect, be possible to determine 

 the rate of compressibility of hydrogen or other gas by direct 

 reference to the weight of a liquid column, or rather of a number 

 of liquid columns, up to pressures of 500 or even 1000 atmo- 

 spheres. For the present it must be understood that, in stating 

 the following results, the pressures in atmospheres are deduced 

 from the apparent compressibility, in some cases of air, in others 

 of hydrogen gas, contained in capillary glass tubes. 



In this notice I will only refer to the results of experiments 

 upon carbonic acid gas when alone or when mixed with nitrogen. 

 It is with carbonic acid, indeed, that I have hitherto chiefly 

 worked, as it is singularly well adapted for experiment ; and the 

 properties it exhibits will doubtless, in their main features, be 

 found to represent those of other gaseous bodies at corresponding 

 temperatures below and above their critical points. 



Liquefaction of Carbonic Acid Gas. — The following results have 

 been obtained from a number of very careful experiments, and 

 give, it is believed, the pressures, as measured by an air-manometer, 

 at which carbonic acid liquefies for the temperatures stated : — ■ 

 Temperatures in Pressure in 



Centigrade degrees, atmospheres. 



35-04 



5*45 40-44 



11-45 47-04 



16-92 53-77 



22-22 61-13 



25-39 65-78 



28-30 70-39 



