l885-] NEW-YORK MICROSCOPICAL SOCIETY. 137 



cavities holding water, and sometimes gaseous carbon dioxide, 

 its liquid, or all three, in the same thin section. 



For the exact determination of the temperature of expansion 

 of the liquid in these cavities, many instruments have been de- 

 vised, all belonging to the class called warming-stages. In these, 

 recourse is had sometimes to the use of a current of heated air 

 or of heated water, or to the conduction of heat by a metal plate. 

 Most of these are extremely inaccurate, often complex, and un- 

 trustworthy, and it may be owing to this cause that Brewster ob- 

 tained, for the critical temperature of the liquids in quartz, 

 results of the very wide range between 20° and 51° C. As only 

 the specialist in lithological investigation will ordinarily have 

 recourse to such apparatus, it will be sufficient for the purpose 

 of this paper to refer simply to a review of the subject, already 

 published,^ and to the description therein contained of a simple 

 immersion-apparatus which I have devised. Brewster, Sorby, 

 and Hartley had used the same principle, while they employed 

 the method which is indicated in the following language : " To 

 determine the critical point of the new fluid, immersing the slide 

 in water of known temperature, removing, wiping it hastily, 

 placing it on the microscope stage, and instantly examining it, 

 seemed preferable to any other mode of operating.'"" I obtain- 

 ed, however, with greater convenience, far more accurate results 

 by means of an apparatus permitting the slide to remain under 

 observation, immersed in a layer of water on the stage of the 

 microscope, and continuously warmed by a current of air from 

 the breath of the observer, or, if necessary, by the conduction 

 of heat to the bottom of the vessel from a small flame at the 

 side of the stage. By this means an accurate determination of 

 the actual temperature at which a fluid inclusion expands into a 

 gaseous state may be obtained in a few minutes to the one-twen- 

 tieth of a degree, centigrade. 



The simplest form of this apparatus, which is inexpensive and 

 can be fitted up by any microscopist, consists of three parts, as 

 follows : — 



I. A shallow glass tank, such as may be cut off the bottom of 

 a chemical beaker, of sufficient diameter for the slide to lie 

 within it, just immersed in a thin layer of water, but separated 



•Jour. Am. Chem. Soc, Vol. IH. ; Am. Mon. Mic. Jour., 1884, pp. 189-90 ; Proc. 

 Am. Assoc. Adv. Sci., 1884. 



"Hartley : Jour. Chem. Soc, London, 1876, p. 139. 



