On the ElastieUy of Carbonic Acid Gas. 308 



off about one-half, a distillate is obtained which becomes milky on 

 the addition of water. On standing some time, a reddish- 

 coloured oily layer settles to the bottom of the flask, which was 

 washed with watei-, dried over chloride of calcium and analysed. 

 Numbers were obtained which led to the formula C^POH : it 

 is iodoform, C' P H, in which one equivalent of iodine is replaced 

 by oxygen, C^ (P 0) H. Its boiling-point is 181° to 182°, and 

 it crystallizes at —6° C. Its spcciflc gravity is 3"345, the high- 

 est of kuown organic bodies. Like mercury, it does not adhere 

 to glass. AVhen freshly pi-cpared it is colourless, but soon 

 becomes brown from separation of iodine. It volatilizes with 

 the vapour of water. Heated in a scaled tube with potash, it is 

 resolved into iodide of potassium and formiate of potash. 



XXXVIII. On the Elasticity of Carbonic Acid Gas. By W. J. 

 Macquorn Rankine, LL.D., F.R.SS.L. ^' E., Regius Pro- 

 fessor of Civil Engineering and Mechanics in the University of 

 Glasgow. 



To the Editors of the Philosojihical Magazine and Journal. 

 Gentlemen, 



IN a paper by Mr. Waterston in the Philosophical Magazine 

 for this month (March 1858), page 221, 1 find a formula of 

 mine for the elasticity of carbonic acid characterized as "pal- 

 pably erroneous." 



That formula is one of a class of formulae for the elasticity of 

 gases, first proposed by me in a paper which was presented to 

 the Royal Society of Edinburgh in December 1849, read in 

 February 1850, and published partly in the twentieth volume of 

 their Transactions, and partly in the Philosophical Magazine for 

 December 1851. Having from time to time revised the empi- 

 rical constants contained in those formuhe, so as to adapt them 

 to the advance of experimental knowledge, and having especially 

 altered my estimate of the absolute temperature of melting ice 

 from 274°-G C. to 274° C, I communicated a revised formula 

 for carbonic acid gas to the Royal Society of Edinburgh in a 

 paper which was read in February 1855. An abstract of that 

 ])aper, containing the formula with its constants, has appeared in 

 the ' Proceedings' of that Society : but the paper containing the 

 detailed comparison of the formula with experiment is still in the 

 possessionof the Society, unpublished; and I therefore beg leave 

 to send you, annexed, the formula itself, extracted from the Pro- 

 ceedings of the Royal Society of Edinburgh for 1854-55, pages 

 280 and 290, together with the comparison between some of its 

 results and those of M. llegnault's experiments, taken from some 



