170 Transactions of the Royal Microscojncal Society. 



II. — The Identification of Liquid Carbonic Add in Mineral 



Cavities. 



By Walter Noel Hartley, F.C.S. (King's College, London). 



(Read before the Eoyal Microscopical Society, March 1, 1876.) 



Plate CXXXII. 



In 1822 Sir Humphry Davy* investigated the contents of fluid 

 cavities in rock-crystals from different localities. His researches 

 showed that in almost every case the liquid was nearly pure water. 

 About four years ago I bought from Mr. Norman, of the City 

 Koad, a microscopic slide of quartz with fluid cavities. One good 

 sized cavity was readily seen with a 2-inch objective ; it exhibited 

 when under the microscope the shape and appearance of Fig. 1, 

 PI. CXXXII. Its entire length was ^\ of an inch, and its average 

 breadth -^hj inch. The liquid at once recognized is indicated by h. 

 Being acquainted with the experiments of Cagniard de la Tour, 

 I resolved to repeat them with this specimen, and therefore, 

 proceeding cautiously, warmed the sHde over a lamp, until it was 

 just too hot to be touched with comfort. On examination, the 

 liquid, to my surprise, was not to be seen, and the cavity under 

 these circumstances appeared like Fig. '2. As the temperature to 

 which the fluid had been subjected was but little above that of 

 boiling water, I concluded that it had escaped from some minute 

 and invisible opening ; continuing, however, to observe the object 

 until it became cold, I was gratified to see a sort of flickering 

 movement within the apparently empty space of the cavity, followed 

 by the replacement of the liquid, as at first. The extremely 

 low temperature at which only the substance assumes the liquid 

 state, made me at once desirous of ascertaining the exact conditions 

 under which the liquid is dissipated and reproduced; for the re- 

 searches of Professor Andrews,! " On the Continuity of the Gaseous 

 and Liquid States of Matter," have told us that at a temperature 

 of 88"^ F., or 30'' -92 C, liquid carbonic acid becomes a gas, and a 

 pressure of even 300 or 400 atmospheres will fail to condense it to 

 liquidity. This temperature is called the critical point. To deter- 

 mine the critical point of the new fluid, immersing the sHde in water 

 of knowm temperature, removing, wiping it hastily, placing it on the 

 microscope stage, and instantly examining it, seemed preferable to 

 any other mode of operating, and although other more promising 

 methods have been tried, the results obtained have been less 

 accurate. 



1st Experiment. The liquid in the two cavities had disappeared 

 completely at 36^ C. ; the cavities appeared empty, but the liquid 



* 'Phil. Trans.,' 1822, p. 367. t ' Cliera. Soc. Jmirn.,' 1870, p. 74. 



